would it hurt the amp to change a 12ax7 ( ecc83)for a 12au7 ( ecc82 )( 19% of the gain ) 12ay7 ( 41 % )

thank you .
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You usually lose drive aswell as volume when you replace a 12AX7 with lower gain tubes
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I have a Mesa Boogie Subway Rocket which I replaced all the 12ax7 with 12au7, it's much quieter (so quiet in fact that I may have to switch back). However, it sounds beautiful in my opinion, much less aggressive than before, which suits me.

Au7 and AY7 both have less gain, so it'll be mellower. Unless you want metal, you can probably conpensate by just turning the gain up higher.

Another option is a 12AT7, but those can sound like crap when overdriven (too much grid current), so I recommend against that unless it's either going into the LTPI slot or you mod the amp to account for the grid current.

Another option yet (and my personal favorite) is a 12AV7, which is probably in the realm of a 12AY7 or 12AT7 for gain (I can't recall ofhand, just know that it's more than a 12AU7). It draws 350mA of heater current instead of 300mA, but I doubt they speced the power transformer close enough that the extra 50 or 100mA would make a difference.

...if you do try 12AV7s and your PT blows up, I warned you though...
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I like using the 12AU7 in the phase inverter tube slot so I don't get crossover distortion. You will have no problem using those. Just less gain and therefore less distortion.
I replaced the ax7 in one of my pedals with a au7 and i didn't hear any less "distortion" but it's quieter and the highs are smoother and the bass is tighter.

ott, im no help.