Getting back on subject, I decided to try to evaluate the noise I was hearing. I hooked up one of my dynahi boards for the other channel and took a look at things on the scope. Both channels (FET/BJT) had some noise on them, so inconclusive. Since I don't have a function generator, I was using a portable CD player with a disc of hi quality test tones. When I unplugged the AC adapter, the noise diminished noticeably, but not completely gone. From what I recall, the dynahi was quiet when I was testing it. I'll need to recheck this with both channels to verify and make sure I'm not chasing my tail.
Since my previous (4 years ago or whenever) testing had shown that the FET amp board went nuts with the crappy headphones I have in the shop for testing, I have had these plugged in most of the time.
I replaced the feedback resistors with the original 1K/100R combo. Now things got interesting. Without the source plugged in (RCAs), I was getting a sine wave on the outputs (both boards!), at all volume positions other than all the way down (full attenuation). I did not determine what the frequency was, but strangely it wasn't really audible on the phones.
I am theorizing that the FET board is oscillating and this is feeding back thru the PSU? Not sure how rational that is, but that is all I have to go on right now as there was no input plugged in, it remained with or without the headphones plugged in, and the only thing common besides the pot is the PSU.
Next time I look at it I will verify that it is behaving consistently, and then go back to the 100K/10K feedback resistors. Maybe see what 10K/1K does as well.
Any other suggestions?
EDIT: I asked Todd to move this thread from the Casino to the DIY forum, and he obliged in doing so. If there are objections to this let him know (or me).