Thank you Peter, that is as always very helpful.
Since I have another project I can use the filter on, I will likely build it anyway, and I may do some listening with it connected and not. (Though first I will listen for a good long time without it.) As you say, one must always be very careful, especially when thinking about whether one is hearing an improvement and not just a difference.
Luckily, I'm not inclined to listen for stuff like "more bass." That way lies madness!
I always think, "Who ever walked out of a Pavarotti [or Stones, who I've actually seen, Pavarotti not] concert raving about 'more bass'?" I listen for more "reality," elusive as that idea is. But I know the sound of a real acoustic guitar, piano, or Jordi Savall playing the viola da gamba. And even more, I know when listening to something like that drum track you sent around a while ago that musicians play
with each other, and if you hear good musicians who suddenly don't seem capable of playing with and off each other in a way that creates drama and joy, then something is quite wrong.
But hey, I am as capable of making mistakes as anyone else (maybe more than most!), so I will be careful if I decide to do comparative listening.