I play through an old and poorly maintained (second, possibly third or fourth hand) Marshall Valvestate 80watt combo. It's got it's share of dirty/broken pots and stuff, but I can deal with that. The problem it's started showing recently is whenever I play with distortion.

I use distortion pedals rather than the amps crunch channel, and it's never been a problem until a few weeks ago. Basically, we all know that distortion pedals will buzz when engaged but not playing right? Well the buzz has now become a high pitched squeal almost like feedback (higher gain the pedal, higher pitch, and the highest one does feedback bloody loudly) but then even more recently there's this constant noise in the background even while playing now?

I've got a noise suppression pedal which goes at the end of my effects chain, I know it's not a fault with the pedals, it must be the amp. Any diagnostic opinions? It sounds to me like it's the tube inside (there's just one) and a friend thinks I could just replace it, but I don't know much about amps so I thought I'd ask
PRS SE Chris Robertson
PRS SE Angelus Custom
Yamaha SF1000 (Both of 'em)

Laney L20H Lionheart
Marshall 1936 w/ Eminence

Rather large pedalboard..