On one leg or two legs?

Hevac1,
The two pole 60 amp breaker will trip just fine if there is an overload or short circuit on just the one pole of the breaker. It is designed to do so.

I assume the reason Maril555 paid to have the sub panel installed was to keep his audio branch circuits somewhat isolated from the loads in the main panel.

If you look at the pictures, Maril555 supplied, you will see he had plenty of room in the main panel for the 5 branch circuits.

Your post got me to look at the picture of the main panel again and that reminded me I was going to ask Maril555 why he didn't have the electrician move the two GFI breakers, across from the 2 pole 60 breaker, to another location in the panel.

Don't know where to begin, well:
I was hoping to find interference from my PC or cable, but no luck. I disconnected and unplugged PC, cable modem, wireless router, switched off all breakers, except the power amps- hum is still there.
Tried different speaker- still there
Tried different power cord- still there
Plugged the amp into a different outlet upstairs, separate from the dedicated panel, through the long extension cord- still there.
Hum, or buzz, is very constant in nature and nothing seems to change it. it does sound like a ground loop buzz to me, more, than anything else.
Again, L channel amp hums more, than the R channel with everything disconnected from it.
Hevac1, thanks for your ideas, but in lieu of the above statement, I don't think compatibility of the different components is the culprit.

Jea48,
I don't know why I didn't ask him to move GFI breakers (they are the ones with yellow markings on them?).
Where do you think they should be in the panel?

Did I mention how frustrating that is ?

One last thing- I switched the amps left to right, and the hum moved too. I don't know, maybe it's inherent to that amp? At any rate, hum is there.

This question probably has been asked, but I missed the answer but if not, are you running balanced? or single ended?
If single ended have you put in the shorting pins?

Ozzy,
I run them balanced