Leif - you might do youself a favor with this :
Try to lend something like a Firface, but in advance of that, get a 10m Firewire cable (they exist, though harder to get - hence you may need to get it from abroad); with the Fireface that works OK. Now, put that Fireface next to your equipment, and compare the 10m Coax with Coax of 1m or so (your Buffalo obviously hooked to the Coax).
After that you (most probably) will know that what you are up to now is not the way to go. I know ...
Or you don't do that test, and you will never know what could be achieved. Also good.
10m SPDIF Coax is really too long. Toslink I don't know, except for that it is not good in the first place (conversions, jitter).
Lastly, keep in mind that comparing SPDIF with USB is a kind of apples and oranges in the first place, because both won't have the same conversion means at the DAC end. For your DAC this is "as is" of course, but it doesn't mean that SPDIF or USB by itself is better. Then of course for your one and only DAC the best result counts.
Peter
PS: With my USB DAC I just used 11m of passive USB cable (actually 4.5m + 4.5m + 2m) but this is a self powered DAC.