First off hi every one. Ok now to the questions I bought a used mesa 50/50 rack mount power amp not sure what the year is but it has the red light on it not the blue one that is the newer model ones. It got low in volume at my last gig so I figured tubes so I replaced v2 and v3 with some used mesa 12ax7 tubes and just channel a's power tubes with groove tubes number 4's matched just to see if it would fix the problem well it did it is good and loud now so now I am going to replace all the tubes with new ones. All I use this for is good clean power as I run a GT-10 through it into a pair of vintage 30 cabs so what would be the best tubes to replace them with I want to stay in the tolerance mesa has on it I don't want to mess with the bias it had 6L6 GC Mesa tubes color code yellow's in it. Thanks and if any one knows any mods for this amp please let me know that also thanks.