I have a Marco LE LM2, just developed this problem too. It takes approximately 30 mins to come up to volume form a 'cold' start/power-on. Once it's at volume it behaves fine - retains full volume if switched on and off, but back to same problem the next day. I can 'warm it up' ready for a gig but.. erm, doesn't exactly fill ya with confidence.
Can anyone help in regard to MarkBass' point of contact in the UK?
I'm about to send it to my local tech if not..
Apologies for dredging up the thread.. first info I've seen on the net on this problem.
Did you get this resolved OP?