So, I've started dabbling in basic electronics repair, and one of my recent projects was my Rocktron Velocity 150 guitar amplifier.    Someone on this forum previously linked to these schematics of the Velocity 300 which is nearly identical to the 150, only with a beefier power section & cooling fan.

http://www.audunmelbye.no/wp-content/uploads/2011/04/rocktron-velocity-300-main-schematic.gif
http://www.audunmelbye.no/wp-content/uploads/2011/04/rocktron-velocity-300-power-section.gif

One of the op-amps had gone bad causing the volume & tone controls to react very noisily with even minor adjustments.  I fixed that, and while I was in there, I went ahead and replaced all the TL072 opamps in the main signal path with lower-noise NE5532As since the amp had always had a low level hiss I was hoping to eliminate.

To cut to the chase:  the amp is now working mostly fine, but while the hiss is gone, I've now got a persistent hum in both channels.  The hum is there even when the volume is all the way down, though it does get a bit louder as I turn the volume controls up.  The hum is easily audible even with my poor hearing, so it's definitely well beyond expectation!

I heard this kind of problem could be caused by bad filter caps, so I bought an ESR meter and tested them, but they all tested perfectly OK.

So, now I have no idea what to look for next and am hoping for advice. This is a learning project for me, so I don't mind investing more in this project than I will get out of it!

thanks in advance.