Yesterday, my young lad and I put the Dada electronics revision kit through the 33, plus squirted cleaner into the knobs, switches board slots etc. We also checked everything for dry solder joints.
Well the revision definitely improves the sound!! The stage is big now and really detailed. I know base and treble have improved, but its the mid range that i'm particularly chuffed with, so much more detail now off vinyl and cd.
So far no more problems with speaker channels. However since I tightened the card slots last time it hasn't happened either, that said I have been careful not to change the filter settings (the other suspected cause) ... time will now tell if its fixed!
On other matters, I'm going to put a Dada kit through the 303 now, its hard to resist!
P.s LPspinner, thank you for alerting me, i'll pull the switches apart if all else fails...