I have two different JMP's that have fairly loud hum, depending on where the volume knob is set. Seems it is on both "volume II" channels. At zero, there is hum, but turn it up to about 6-7 and the hum goes away. On one, it hums except for a narrow window at about 6. Filter cans are all replaced a few years ago, and the amps sound great with all NOS Mullards. Bad coupling caps? Seems odd that both amps are affected by volume two. I am pretty good with a soldering iron, and have done a fair amount of wiring/parts replacement over many decades, and can easily read a schematic, so I wanted to know what to measure, and where to start or look for the issue. Perhaps this should be in the Workbench, but I figured there was more of a chance of any of you having the same experience with these old amps. Thank you in advance.