Help!! Amp hum!!

I have a solid state amp that gets a grainy hum or hiss when it's turned on.

It is NOT a ground loop issue (I've experienced those before).

So, when I have my amp turned on, there's an audible hiss that I can't get rid of. It used to be silent (When I first got it, used) and the noise crept in. I've taken it to multiple houses, tried different outlets in the house and the same sound is there.
I've listened through the headphone out and through a normal cab and the sound is worse through the cab.
I've tried different basses, cables, and no bass/cable.
I've tried turning the gain to 0.

Nothing. I've tried everything I can think of.

Any idea why an amp would have this happen?

I have an old combo amp that does the same thing but much worse, but it was sitting for a long time before I got it, and presumably the dust or something is what I thought caused that 7 years ago when I first started playing

 

Thank you! I wasn’t sure which forum would be best

 

Any idea how to move my forum from one to the other?