buzz/hum what is going on here?

I have a pair of monoblocks in a 2nd system (Counterpoint Sa20s) and run them off separate dedicated circuits, use PS Audio Power cords, have the grounds cheated, etc etc. If I leave the amps on over 8 hours, one channel(monoblock) begins to hum. The hum does not get louder as more volume is added. It is not audible over most music, but can be heard during quiet passages. If I shut down the amp and restart within a short time, it is still there. If I wait overnight, it is gone. Im puzzled.
Does it hum at the amp or at the speaker ? If it's at the amp it's possible that as the amp warms up a parts expand mechanical tolerances change and the transformer starts to hum. You might be able to fix this, but unless it's really loud and annoying I'd just leave it alone.

If it's at the speaker then that is very interesting, and might warrant a call to the manufacturer.
it is at the speaker. i ran the amps for 3 hours today and the hum was lower than it was last night after being on for 8.
These are tube amps ... unfortunately I have no experience with tubes ... but I have heard others comment that hum can be caused by tubes ... is it easy to swap the tubes between the amps ? You might isolate a bad tube.
There are a few things that can cause this kind of hum.
If you have gone through the obvious things like cables and the rca jacks on the amp are not loose and swaping the amps to see if it is the preamp causing the problem then this is what i recommend.
Unplug the amp
let it sit for 24 hours
Take a volt meter and check to see if there is still dc voltage on the big filter caps.
Carefully take a good size screwdriver and tighten the screws down on the large filter caps.
They get loose over time and I have fixed the hum problem many times by doing this.
It is worth a try.
Hope I have been of some help