@lensman, I dunno about the 6581 in the c64c sorry. I might give it a go if you're too scared! Unbeleivably I found the other night I've got another c64 i didn't know I had so i've got spares ;-)
BTW on the quietness thing. I found that with a working 6581 in the slot next to the metal plate it DID work. IMHO The sound was grotty/odd phasing and only in the left, and there was still CPU noise, but it did work and the audio was clearly louder than the CPU noise. The first SID I tried was as quiet as you describe on here, but I found that pin 14 on the SID (and on the slot on the c64) was corroded and didn't work in a real C64 either.
@amigakit - I'd watch it! this is clearly a very early peice of hardware that isn't yet ready for release. I am exceptionally disappointed with mine, especially considering the price. I would expect a high return rate if I was selling them!