Hi Peter, thanks for your reply. I have done as you suggested and posted the original email in the place where this was before to give the context.

Yes it is now July 2018, and I finally completed initial build and assembly of the IPL S4TLs I ordered in May. Scroobius (thanks mate) advised us to choose the S4TL, as did Ivan from IPL, and we decided it was good advice to follow from the only people we knew with first hand experience of both S5TL and S4TL speakers.

The S4TLs use the Hivi 8" driver and the Fountek CD 3 Ribbion tweeter. I wanted these to replace my old 1997 TDL RTL2 speakers which are a similar sort of Reflex / Transmission line design. I was looking for more punch in the gut and better accuracy in low bass, and I hoped for greater clarity in mid-range and treble. Ivan has been refining this S4TL design for many years, maybe 20-25 years now, and perhaps now the cross-over and units selected are as near to perfectly matched as is possible. I followed his design exactly.
 
I have played them now for about 5 weeks and tested them against different speakers in different rooms. For comparison, we are talking about my TDL RTL2s, and a pair of approximately 2000 vintage Tannoy dual monitor gold HPD 15" speakers in 150 Litre Tannoy enclosures.
 
The IPL speakers are amazing. Scroobius mentioned "Revelation", and that's what these speakers do, they reveal so clearly things that were only hinted at by other speakers, and even earbuds.
 
They have such tight control over the bass, there is not a hint of boom, but the low bass is there. All the way down low, it plays musically, with no hint of the "sameness" boom you get from speakers with less range at the low end. Anything which previously sounded like it was at the low limits of my TDLs or the big Tannoys is now delivered tunefully, calmly and distinctly by the IPLs.

Massive Attack's "Protection" album is full of soft and soggy and very deep bass.  The IPLs maintain complete control over what really is very bass-heavy stuff, while providing such accuracy in mid range and treble that we were immediately hearing things we had not really noticed before. Again, the bass notes are clearly presented as separate, musical and melodic notes, always.

Police's "Walking on the Moon" (re-mastered) has such incredible attack on the pedal drum, it sounds like it is being hit with a wooden stick, not a soft pedal - there is a real crack to the impact that really belongs - you can hear it that way on headphones and earbuds. Sting's high tenor is all there, but with no hint of strain.
 
They are just as revealing of treble and mid-range. I heard fine and fast  cymbal work in "Sultans of swing" that I had never noticed, even on my earbuds. This occurs at the very end of the track just a few seconds from the end where the final short guitar solo is at its peak, yet there in all that loud sound is the super-quick, (maybe treble-time) tapping of that fine cymbal sound. Of course, now that I have heard this on the IPLs, I can tell that the sound is present in the earbuds and those other speakers, but I only hear it now because I know it is there, and where to listen for it. The clarity yet smoothness of the tweeter is amazing - I cannot imagine improving on that tweeter sound with these speakers - that balance seems to be exactly right - accuracy and attack without ear-piercing aggression or undue sibilance.

The mid-range is controlled and delivered very well by the Hivi 8" unit and the crossover appears to be invisible. We were able to listen to "difficult" music that screeched at you on other speakers - with the IPLs it sounds pleasant and controlled even at very high volume. We had a fun time dragging out "difficult" songs that we loved, and playing them really, really loud, without needing to turn the volume down when the previously screechy section of the song arrived. Prior to the IPL test, we always reduced volume for these passages.

Words to describe these speakers... revelatory, controlled and authoritative, but most of all, just plain musical. There is no colouration that I can hear, this is particularly obvious when listening to news reports and other human voice soundtrack from TV or movies - the speakers do not have any boxiness or bassy colour to them.

We have played these very, very loud, and also very soft in two different rooms. Both are big enough to hold these speakers, but they have quite different brightness. I have found that their positioning is both important and variable, it is worth moving them around a bit as even an inch or two  can make a big difference TO ALL ASPECTS OF THE SOUND.
 
In my room, the speakers work well in two places - 12" away from the back wall, (not 10" or 15") and funnily enough, they work fine just 4" away from the back wall. Weird, but then again our room is a weird shape with a very high ceiling. Currently they are sitting 4" away (which is the nicest for WAF) and sound just about as good as they do at the 12" position. In my friend's room, they were also fine at about 12" from the back wall.

Am I happy? Well yes, these speakers have delivered everything that I was hoping for. Clarity and musicality, also the "feel" of very much bigger units than they actually are. There is a certain "Grandeur" to the sound of the 15" Tannoy Dual monitor golds, which while not necessarily accurate, is very attractive, and these IPL speakers go incredibly close to replicating that same sense of grandeur in almost every track we tried, but with a sound stage and an accuracy that is obviously far greater than that delivered by the delightful Tannoys.

I took advice from Scroobius who has built 3 IPL kits, and I stuck rigidly to Ivan's design. The build was not as easy as it is for those with full kits from IPL. Because I live in NZ, I could not afford to ship the 50Kgs of MDF here. I had help with cutting MDF, but this was not a CNC machine - just a big table saw, and I had to cut the various holes etc myself. Nonetheless, the build was easy enough, the hardest thing for me being the soldering of the cross-over, and the fitting of the bracing and the foam. These were not really difficult to do, but it was fussy work for somebody with such limited DIY and electronics/soldering knowledge as myself.
 
I have seen comments from some people suggesting that these speakers have some lack of this or the other in the sound, or that they are "woolly" but I really think that must be room issues, incorrect stuffing, or placement issues (it makes a GIGANTIC difference with these speakers). Maybe earlier versions were not as well matched with cross-over and stuffing as they are now. I say this because the accuracy and separation of the instruments with these speakers is incredibly accurate, more so than our other speakers. The sound stage with earbuds is obviously more distinct, but the sound is not otherwise noticeably different, and these IPLs truly deliver something close to earbud accuracy and "crack" on drums,  and all other instruments, but with real gut-punch IF IT BELONGS. The IPLs do seem more revealing than the earbuds, but I do not really understand how that could be possible. I can hear the sounds on the earbuds, but had not noticed them before hearing it on the IPLs.

I have found that at times the IPLs sound LESS bassy than the other speakers we compared them to, but that seems to be at points to do with resonance and the limitations of the one-note sound at the lower end of those speakers with less range. The IPLs always sound more controlled and tighter. There is no point where the IPL speakers boom at you. So to our ears, they are the exact opposite of "woolly". Also, I may be just imagining this, but it seems to me that they did seem to get a little better once they had done a bit of work, though they were obviously and extravagantly amazing the very first time we connected them.

Again, am I happy? Yes, but these things show EVERYTHING, so trying to listen to Layla (Derek and the Dominoes) is now really hard, because you can tell just how awful the recording is, and that is the same for a lot of the music I love. Be warned, these are so accurate that they will show all the badnesses in your system, from source to speaker. I did not fully comprehend this warning when I read it myself, but it is oh so true. If you want hifi, then these are for you. If you want to listen to poor quality recordings with the same pleasure you did before, then forget it and stick with what you have.

Sorry, I just re-read this and thought how arrogant it sounds - forgive me. I mean to say that these speakers are way better than I could otherwise get for the same money, with high quality units in a sophisticated and well-proven design. Of course most of you on this forum will have better sounding speakers than these anyway, but probably not at the same amazingly cheap price. I wanted to share my experience with these in case any others of you are considering an upgrade of old equipment, and wondered what these IPLs sounded like to us, and our wives. The wives are very impressed, so we are happy with our choice. Thanks again to Scroobius for his help with this.

Scroobius, congratulations on your Orelinoi speakers, they must be lovely when you have the room and positioning tweaked to perfection.
I am not jealous of you, as these would not suit our purpose, though I am envious from point of view of how good they must sound.

Sorry to be bothering you with this giant email, but we have no chance of hearing these before we buy, and for both of us, the decision is a long-term choice for us.
These will probably be the last speakers we will ever buy, it is a luxury purchase for us sanctioned by our wives.
There are two, possibly 3 of us who want to build these kits, S4TL or S5TL. We are all 50-60 years old, and our musical tastes come from the late 60s to 80s with some other stuff later than that. This includes Floyd, Mark Knoppfler, Massive Attack, Classical, some jazz, lots of rock and blues, and a mix of everything else including (shudder from me) Michael Jackson and Tina Turner stuff that the ladies like to dance to. Ugh. Also lots of other stuff.

I currently have TDL speakers, the little RTL2s, which I bought in 1997 bass only down to 40-45.
Harry has Tannoy 15-inch dual monitor golds in small-ish Tannoy enclosures – bass only down to 40-45.
Garth has 2 different sets of B&W speakers, neither particularly high-spec.
We ALL have wives with a love of music, one of whom likes to play it loud and dance. We love rock and blues, some classical – I have a fondness for classical and in particular, I like organ music.
We all miss the deeper frequencies that live between 20-45Hz, and we all want them.
We all want to feel the music, playing loud or playing soft.
We ALL like to feel the visceral impact of deep bass, but who doesn’t if it belongs?
We all like natural sound for voices and instruments.

None of us have the budget to buy commercial speakers of the quality of the IPL range, and we are excited about the project and the prospect of truly great speakers.
We are trying to choose between S4 and S5.

I have modelled the S5 size box in our sitting room, and it looks fine. The room is over 27 feet long and around 14 feet wide at the narrowest part, being very high ceilinged with a sloping roof between 11 to 18 feet high. Speakers will be placed approx 4’ from the side walls, and about  7’ apart. I sit about 17’ from the back wall on the long axis, with the speakers firing the length of the 27’ room. My TDLs work well here placed about 14” away from the back wall.

The main part of Harry’s listening room is 40 feet by 16, also sloping ceiling with height around 10 to 16 feet, his listening position is normally around 16’ from the back wall.

We like hifi sound, but we love music, and sharing it with our friends. We sometimes take the speakers outside and let them breathe. We all take turns to play one of our favourite bits of music. The speakers all sound WAY better out there than they do in a room – bass is cleaner and open, no nasty room effects, and the bass does NOT sound reduced in either of our sets of speakers when we use them like this. This is very surprising, but true.

We are NOT over-driving the speakers here, they are just “normal” loud. We may be pushing as much as 15-20 watts RMS into them, but not more than this. Seating position in this mode is between 12 to 18 feet from the speakers spaced so that they sound best. I have never harmed my TDL RTL2s (6 inch bass/mid drivers) with these amplifiers, and they are loud enough at those levels without taking the amps anywhere near to clipping.

Harry is certain that if he builds one of the kits (he will make that decision after hearing my build) he will build the S5, not the S4, and I would be sad beyond all belief if his S5s sounded noticeably better than the S4s if that is what I build.

We really need the OMG response for these speakers from our wives, as otherwise they will scorn us for the rest of our born days. They will expect and demand to buy any number of expensive shoes and handbags from now onwards, at great detriment to our sanity and overdraft.

For our own sakes, we want to build the best sounding loudspeakers that we could never afford. I honestly believe that both S4 and S5 are able to deliver clean sound in very low bass frequencies, but my sense of the many reviews and projects I have read indicates that the S5 is possibly a step above the  S4 in really low bass reproduction. Also, at louder volume, my guess is that the S5s will hold the midrange together better, whereas I almost expect that S4 at louder levels may suffer slightly in the important 1K – 3K range – but maybe I am wrong on this.

You have heard both speaker, and know the differences between them. Let me ask some questions for you to answer with your own valuable opinions, and we will at least have more information on which to base our decisions.

So, questions.
Naturalness of voice. Is this equal between the two.
Souza marching band bass drum, Biggest Orchestral Tympani, Big kick drum from rock type music.  Is this sound the same impact wise and accuracy for S4 and S5?
Deep organ notes  - does the S4 have the body and power of the S5 at normal listening levels?
Is there ANY noticeable dip in the S4 frequency response between 25 Hz and 50Hz.
Is there ANY noticeable dip in the S5 frequency response between 25 Hz and 50Hz.

Is the mid-range of the S4 as un-fussed at louder levels as it is at normal inside listening levels.
Is the mid-range of the S5 OK? It seems on spec as if the mid-range driver is too inefficient to match the bass driver and tweeter, but I believe that in the real world it all works together perfectly.
What about the S5 tweeter placement – it just looks wrong. I think it should be at the same height as the mid-range if that is possible, not so low down in the box. This business of having to tilt the box backwards just seems like a workaround for a tweeter placement that is a bit unusual in a floor-stander.
Finally the most important question. Do YOU have any major preference for one of these for your normal listening? They will be different from each other, but is the difference quantifiably better in S5 compared to S4? You must like one more than the other for your own music, and I wonder if you prefer S4 to S5, or maybe even one of the smaller sets, like S2?

Honestly I am leaning towards the S5s, but may do the S4s. Harry will almost certainly build the S5TLs if the sound of whatever I choose is as good as I expect.