Well, the first thing I did when I got my Bass 400+ a while back was open it up and clean it out. Dusted all the crap off of the amp and the Mesa tubes and then plugged them back in. I must say that this amp is badly in need of a retube. Five of the power tubes date back to 5-17-97. I also noticed that the preamp tube (Mesa) that is in the furthest left position is called a SPax7. Any clues as to the difference between this and a regular 12ax7? Also, I read somewhere that 12ax7s self bias to the circuitry they are in. Is this true? Why would Bob at Eurotubes be offering matched 12ax7 sets then? Thanks.