I have a Marshall valvestate 8080 combo, it used to belong to my oncle. The amp sounds great on the normal channel (which is Solid State I think) but the boost channel (which has a 12AX7 preamp tube) is very quiet about 10% of the normal channel.

Also I noticed when I played the amp in normal channel the volume goes up and down without me touching it, which is very frustrating when playing in a band setting...

Does anybody know/believe what the issue could be, 'cause I really want to fix the amp,
it has an emotional value to me.

does it fuzz with reverb on? open it up carefully, unplug the speaker and have a look inside, my reverb tank was causing issues like this