I just completely went through this amp trying to get rid of the slight hum it had. New caps everywhere, thermal grease through cleaning and biasing and dc offset. I still have some slight hum and can't track down where its coming from. If I short the inputs I can't hear it. Also if only one channel is hooked up I can't hear it. This makes me think it is solveable. I'll attach my folder full 'o schematics for ya.
https://dl.dropboxusercontent.com/u/3011761/4004 Service Info/004mkIIUpgrade.pdf
https://dl.dropboxusercontent.com/u/3011761/4004 Service Info/4004 board layout.pdf
https://dl.dropboxusercontent.com/u/3011761/4004 Service Info/4004 power supply.pdf
https://dl.dropboxusercontent.com/u/3011761/4004 Service Info/004mkIIUpgrade.pdf
https://dl.dropboxusercontent.com/u/3011761/4004 Service Info/4004 board layout.pdf
https://dl.dropboxusercontent.com/u/3011761/4004 Service Info/4004 power supply.pdf