Thanks Csontos. I determined that the problem is a leaky power cap, called Mouser and got a suitable replacement (computer grade, electrolytic, same diameter, a little shorter). They are easy to replace using screw terminals. I then did talk with Gary (finally caught him in the office), who felt that this is the right approach. Do this first and see if it fixes the problem (humming). I think it should - it's pretty obvious. He said there was nothing particularly special about the caps that they used vs. what I can get from Mouser.
I know that ultimately I'll need to replace the caps in the signal path and it is harder to access them on the boards, and that this requires soldering. That will come later.
Csontos - I haven't heard the 100/100, but the 25/25 is pretty nice sounding.