Well, I did my little SPDIF test. The source was my Asus P4P800. The receiver was my Harman/Kardon AVR 310.
Using the cheap junk I have on hand, running through 52 feet and 3 extension connectors was no problem. I couldn't really hear any big difference between that and the 26 feet plus one extension connector that I normally use (I remeasured the cord I use, so it's 26 feet total, not 18). Adding another extensoin connector and 10 more feet of cheap RCA cord (for 62 feet and 5 connectors) also worked, but I started to hear a few dropouts. At that point, I couldn't find another connector, but I added 10 more feet (72 total) anyway and just held the two ends together with my hands. That also worked, but the dropouts were even more noticable.
I would say that if you can get a single good cable 75 feet long that doesn't need any extenders, you might be OK with SPDIF, but I'd measure carefully, and if you can get away with a 50 footer, use that, since in my test, ~50 feet seems to be about the limit with cheap cord for good digital signal transmission (with my current "test" equipment).
I didn't try an analog signal, but I would imagine that the results would be OK at best with 50 feet, with the results getting poorer and poorer as more length is added.
I just thought I'd follow up.
