my M-60s are supplied straight from the wall. They are not happy running through the P-10! From the point of view of distortion from the M-60's as a function of voltage, would you expect a measurable difference in distortion over the range of 115-125 Volts?You would certainly be able to measure a difference, not just distortion but also power.
It seems that conditioners like the P10 are not happy with higher constant loads on them- they work fine with higher power solid state amps that have otherwise low quiescent current draw, but tube amps with larger draws (for filament circuits, perhaps also class A) seem to cause them trouble. For that reason we usually recommend running the amps straight out of the wall.
If you want a really nice conditioner, a company called Elgar used to make some that are quite impressive.