On one leg or two legs?

I'll try radio

I would take the noisier of the two amps back to your friends house and have him connect it to one of his speakers...... Take along the power cord and the speaker cable. Take along your multimeter too.

If it does not hum/buzz there, &^#@$%#@, Check the voltage at the wall outlet he plugged the amp into. See if the voltage is lower than yours.

Are these amps self biasing?

Actually, I just wanted to post about something I did last night-
I moved amps L to R, and the hum moved as well, but I have also noticed, that a level is lower on the side of a humming amp (center image moved to the opposite side),
I double- checked moving it back, there is disbalance.
So now I'm strongly suspecting there is something buzzing inside the amp. I'm going to ask my friend, if he wants to take it to BAT for check- up.
Question- what internally can cause this kind of buzzing?
Cap, resistor, bad tube?
Amps are self biasing

That's very interesting. So the amp with the more severe hum/buzz also has lower gain. It could be due to a lot of things, but I suppose the most likely culprit would be a tube. Perhaps before taking it back to him it would be worthwhile switching tubes between the two amps.

Regards,
-- Al

Al,
Yes, that's what I'll try tonight.