Forgive the obvious questions, but just to be sure:
You didn't change strings from nickel or steel to nylon tapewounds, adjust pickup height, or use different cabinets with 8 instead of 4-ohm impedance? Or boost the VLE knob? Sorry, had to ask.
The Markbass site isn't good about listing authorized service centers (I have a Little Mark 250 as a small rig head), but assuming you are still under warranty, you really should seek one out. I live in the Boston area and I've never noticed Markbass at Mr. Music, and I wouldn't recommend Daddy's or GC. We don't really have too many other options around, do we?
If you can't find an authorized repair and you are off warranty, consider AZTECH Electronics, 246 North Beacon Street in Watertown. Tom is a good guy and does good, honest work, but he's not inexpensive. Electronics work that involves extensive diagnosis can really eat up hours and dollars, but he'll be straight with you and cap things before they get past your limit (although you may not end up with a solution if you can't go far enough - just the reality of this sort of thing - explain up front what you can handle). I'm taking an old cab there tomorrow and I can ask him about Markbass if you'd like.
Mr. Music will also do amp repairs, but they referred me to AZTECH when I explained my problem.
