I'm the M-Pulse buyer. And to confirm, about everything that can be troubleshooted has been troubleshooted. Different basses, different cables, speaker leads, kettle leads, different cabs, different sockets. I've opened it up and there does look like a potential 'dodgy' valve (the silver cap on one is in tact, on the other its whited out) but both glow and warm up and neither have broken filaments. I've run a bass through the FX return to bypass the pre-amp and the power section seems OK, and as mentioned above I've tried the trick of running a shorted patch through the effects return to bypass the problem that the BB750 had. Unfortunately the amp doesn't seem to want to produce the volume it should. With the gain at 50%/60% and the master volume all the way up I can comfortably talk over it, which shouldn't be the case! @Beedster has been an absolute gent with all of this, we've agreed the best course of action is to rewind the sale as taking it to a Mesa Authorised repairer is never cheap! There is the outside possibility it's something in my environment however in that environment is an Ashdown CTM300, the Mesa Big Block, a Blackstar Series One, a Marshall JVM410, a Friedman JJ20 and an 800w PA system all currently doing as they should. Tis curious indeed.