Hi I’ve got a JMP 2061x manufactured in 2004. I bought it last year (2019) and it contained the original tubes (It said Marshall on them). It sounded fine but since I play a lot I’ve now changed all tubes (JJ´s) and it came back to life again. But now recently it feels thin in sound, the volume goes up and down randomly. After about one hour of playing it settles. When powering up, it also generates quite some noise, more than before. Even without anything plugged in. I can send a sound file from a power up so you can hear the noise it generates. I couln´t find a way to attach it here. So is there anyone out there who please can point me in a direction what to do? Since the tubes are new I think it must be something else that is in need to be replaced. Thanks in advance!