I am looking for help diagnosing a hum from my favorite amp of all time. I am going to list all the facts and hope you will read them so we stay on track. 1: Hum is 60 Cycle (Hz). I verified this with a tone generator. 2: Hum is present without bass plugged in. 3: plugging bass into effects return eliminates hum 99% 4: Hum is controlled by volume knob. It's there from about 2 and gets louder as you increase volume. 5: I have eliminated tubes as the cause by swapping each and every tube. 6: there are times where the amp is dead silent and then the hum comes back. >This leads me to think it's an internal connection or something from my home wiring. 7: I have pulled and inspected each and every circuit board and there are no bad solder joints visible. 8: Amp has only about 40 hours play time. but I have had it for 8 years or so. I always make sure to play it every few months or so. I think the longest it went un-played was 6 months. Please give ideas on what I could possibly look for inside. I can add jumpers to grounds if that could possibly be the cause but all ground lugs are tight.