I had a show a few days ago and plugged my rig back in (Mesa Bass 400, pair of 1x15's) today to practice. I noticed that at higher (stage/rehearsal level) volumes, there is some distortion/baffle on the lower notes, like a crappy unwanted overdrive. The higher notes come out clear and sound great. I've tried all manner of lowering my bass volume, lowering the volumes on my pedals, testing out high/low channel/master volumes and it seems like the problem comes at a certain volume. I switched cabs, switched speaker cables. Tested my bass on another amp to see if my battery was dying I thought that my speakers might have been blown from the show because I had to drive them pretty hard to fill that venue, and that might be part of the problem. But on inspection they both looked fine and dont make a flapping noise or anything. Any thoughts? What does it mean when a tube goes "microphonic" ? I dont have a super fancy shock mounted rack for my head and am just using a reinforced regular one. I am ultra careful trasnporting it, but maybe it suffered a tiny impact or two.