A fixed bias means the tubes have to be within a tighter spec. Mesa's theory is to use select tubes and eliminate tech quirks that not only cost you time and money, but can screw up the amp's engineering. If I was set up to bias my own amps, I would be more inclined to consider fudging Mesa's schematics, but I have been spoiled by another fixed bias amp (a THD Univalve) that lets me experiment with almost any decent tube, input or output. I found the Tube Freak link before I purchased this 395, and together with the Harmony Central reviews, figured it was a good investment. My reason for the 395 was that I reached a certain tone I like with my THD Uni, but it is a 15 watt amp and I want to use it as a front end, a pre-amp. Enter the 395. The line out from the Uni is my guitar pre, its speaker out is my stage monitor, and the 395 will drive 4 x 12 cabs. I am trying to reproduce the Class A output from the Uni (and its NOS valves) with the Mesa's 190 watts of bliss. Mesa's tech didn't push their current tube line as hard as I would have guessed, which implied to me that the amp's engineering may have sprung out of yesteryear's more tightly monitored tube manufacturing (NOS tubes were more plentiful twenty years ago). I understand that today's tube production is all over the board, hence the need to bias an amp to match the crappy current crop of cruddy tubes designed for crunchaholics. NOS tubes, especially those designed for military use, were manufactured to closer tolerances. I am taking a leap of faith, assuming the Mesa engineers used high quality, thoroughly tested NOS tubes to substantiate their schematic blueprints. I did not want to smoke this amp by rolling incompatible tubes through it. Eurotubes has some re-tubing recommendations specific to the 395 (that make it run hotter without changing the fixed bias), and Watford Valves has specially-selected tubes (for Mesa's fixed bias amps) in a few in production varieties that they hand-pick. These are a couple of end runs to avoid doing a bias mod. Not to say there is never an alternate, better way. But for now, it's time to roll...