bad sound from marshall superbass...help

[rykus]

hey guy's we we're jammin the other night and running a bass through my superbass, to mesa cab. started making some weird fuzzing and crackling sounds, they sounded less prominent and almost went away with the presence down at 0, i got to it today and found my tech had set the bais really high @ 20-23 watts on my compu bias..... so i lowered it to 12-14 watts. sound persisted, new pre amp tubes..... still there, different cab because i started to think it was a bad speaker... nope. new power tube helped a bit but i really don't like running it with a potential problem...... any ideas? does sound like a blown speaker but more just certian chords or notes, louder with presence up and really anoying at any volume! help me, i don't really want to bring it back to the same tech or take it the 2 hours to the city! cheers