Hum!!

Agree that it's most likely a power supply cap.

Whatever you do don't use George Meyer (I've lived in the LA area since 1977).

Check out:

https://musical-fidelity.com

DeKay

If the amp can be cycled off and on with the hum going away for a while, its not a bad solderjoint.

Tubes can do this if they have a cathode/filament short. But the problem is in both channels so I don't think its that.

A filter capacitor in the power supply could be failing. I've seen them act in exactly this way, and the amp is old enough and they didn't have very good filter caps to begin with, so I think that's the problem. This should be attended to- it will only get worse and could damage the power transformer if the cap is also leaky!

@stereo5 Nope. Dry. As in it has "dried" out over the years from not having the flux flow properly and cracked. Cold is a faulty solder joint from the start by not having the iron hot enough. I said connection instead of joint to help clarify in case the OP was unfamiliar with the terms.

Here’s a lovely story for post holiday enjoyment: A hum appeared in both my subs, which are RELs connected to a single ended amp (Inspire Fire Bottle HO) with Canare wire attached to largish spades to combine the cables…works great. I take the cables apart at the spades and re-do them, look at the Neutrik plugs at the subs, tighten everything, check all the AC cables for grounding issues…basically take the whole gear heap apart…still humming…low frequency just going on and on...go through the amp tubes (there are only 4 on this amp so no big deal)…KT88s swapped out for others, 6SN7GTB swapped for its backup…no change...I get to the rectifier, a 4 month old Gold Lion GZ34 and BINGO! Swap it for a 274B I had in reserve and the hum disappeared. The end.

@stereo5
+1 for wit

call them quick

if it is the power supply caps, they may blow and wipe out the entire amp, maybe some drivers too...

George Meyer, A/V repair expert, located near Santa Monica, or west side, they moved recently. Very good, very reliable 

Good luck,
Bob

Sounds like a dry solder connection. 

I'd give Jolida a call and explain the symptoms. Their customer service is pretty reliable - if a tad bit flaky at times - and they stand by their products.