Ampeg SVT-4 Pro 'always-on'-Humm in the preamp-section (NOT DI)

Hi all,
welcome to my first thread

My Ampeg SVT-4 Pro (about 10 years old) humms continuously, no matter the pre/master-volume or if there's a bass plugged in.
It stops if I go directly into the Poweramp!
(If only 1 cab is connected the right power amp has to be plugged-into to stop the humm of course)



I have only found one old thread, possibly about this:
Constant Ampeg SVT-4 Pro Hum. Any Ideas?
where @Basslan had a similar problem, but the thread stops, before he tested circumventing the poweramp.

I probably won't fix it myself, but I'd like to narrow down the problem as much as I can, before getting the amp to a local tech here in Austria.

Thanks for your help

 

Thank you, that's probably the easiest to start.
I somehow dismissed the idea, because the pre-volume doesn't change the humm at all.