Hello!
I wanna tell you a story.
A few weeks ago, I was gigging my Promethean, and the sound turned to mush part-way through a song. At the end of the tune, I checked it, unplugged everything, tried again, and it was fine.
I didn't give it another thought until JPJ (BC'er, not erstwhile Led Zeppelin recorder player) came around for coffee one morning. He was interested in the Prom, so I gleefully demo'd it for him... and it sounded cack. Washed out and floppy, to be precise. I was horrified.
Turns out something was loose in the assembly behind the input jack. I could ease it gently by wiggling the plug, and depending on how it was jiggled, it would either sound washed out and floppy, or the SHAZAM! punchy Promethean tone we've grown to love.
I took the head apart and had a mooch around, but I'm cack-handed so in the end I took it to a repair chap who fixed it and gave it a going-over for 30 quid.
Thinking back, I remember the moment I'll have done it - I was reaching over the combo one day to pick something up, and I accidentally caught the plug, giving it a shove. It had been slightly wobble ever since.
Err... that's it. Hope it helps.