Thanks everyone. I re-flowed the 6-lug strip joints and the headphone jack and that fixed the main issue. There was some very faint left channel frying bacon crackling sound and RF pickup I was able to notice after that. I decided to just re-flow all the remaining solder joints at that point (really didn't take very long) and now both channels are perfectly quiet, and the amp sounds great.
I do have one additional question - is there a fairly simple mod that reduces the gain somewhat? I'm getting a pretty small amount of volume pot turn to get my HD650's up to a comfortable volume. A little more useful volume knob turn would be nice.
Thanks again