I have an LMIII. It's about 3 years old. I've done at least 250 gigs on it. Love the amp. Last weekend I plugged in and I had to crank it to get any volume. Tried different cables and basses. I usually set the gain on 3 and the master on 4. Now I've got to crank the master all the way up. What's also odd is it seems to slowly gain power some power back over time. Any ideas what's wrong?
IIRC MarkBass set the effects loop to parallel as default so leaves out the normal effects jack trouble. It could be a corroded interconnect cable inside the amp (like a ribbon cable). Other than that I don't have much knowledge of their design.
Try pulling off and reseating your ribbon cables inside. There is very little you can do on your own to those amps otherwise, so if that doesn't work, I'd take it to a tech.
Besides what's been suggested, have you checked the battery in your bass? I know it sounds obvious but sometimes we overlook the obvious.
Jimmy M I pulled the ribbons and reseated them. I'll see if that works on my gig tonight. Bringing the old SWR SM400 just in case. Freddels I mostly use a passive Lull P5 so it couldn't be a battery. The head also did this with my Stingray. It must be something inside. Let's see what happens tonight.