Pardon the 2nd post on the same topic but here's a funny story Caesar Diaz, SRV's amp tech, related about setting SRV's amp up in different venues that makes my same point but in a more humorous way (source: August 2000 Tone Quest Report):
Q. Back to amps…if you polled most of the amp bulletin boards for a “best amp” survey, Super Reverbs would come out on top. Stevie used them for a long time, didn’t he?
A. Stevie used to have Super Reverbs, but they somehow never sounded quite right to him – too much power, you know? He also used to be real superstitious about the number 6. He’d always set the controls on his amps on 6 – the treble on 6, the bass on 6, and I’d back off the knobs with a screwdriver so that when it said 6 it was really on 10 (laughs). You have to do these things.
There's a longer Diaz interview I can't find now* but where he explains further that he was constantly adjusting Stevie's amp settings to tune his rig for every different venue he played because the same amp, on the same settings, is going to sound different in different venues. SRV not only believed that 6 and 6 were "best," for him, it rose to the level of a superstition. So Caesar would be constantly "tuning" the amp to the room. Then he'd reset the amp knobs to show as being set on 6 each, SRV would step up to play that night's gig, the amp would sound fantastic, and Stevie would say to Caesar, "You see - I told you these are the 'best' settings!"
EDIT: FOUND IT!
Rubin, Dave (March 1993). "Gear Guru: Cesar Diaz Reveals the Tech Secrets of Stevie Ray Vaughan, Eric Clapton & Bob Dylan". Guitar Player 27 (3): 109–37.