If amplifier power is not utilized to correct sub driver overshoot etc. Where does the additional power come from ??? It's all one system. The amp constantly EQ's the drivers, takes power from somewhere.
It's not quite like this is a discrete thing the amp does. Servo control is basically the same thing as global negative feedback, which your purifi modules do as well. It is looking at the position or velocity of the drivers and, if it is not where the system expects, adds or removes some input voltage before the amp stage. So it can mean adding more input, which eats headroom, but we are not usually talking about a massive portion of signal. Your purifis could have the same core thing happen - their output is a little weak so the feedback mechanism adds more input, eating some headroom.
The bit about 80% volume clipping does not seem like an issue because *if you ever have to turn the volume on your m33 that high, you have bigger issues*. The question is why it needed to go.up that far anyways. The diagnostic challenge is that the M33 is kinda inscrutable for what it is doing. Ex. I glanced at the manual yesterday and the sun outs and line outs are different voltages, which is really weird. The bigger issue is that it is filtering frequencies, and I would not bet on knowing if they are all defeated at any moment. In retrospect, I worry that your test with the speaker level inputs may have lacked bass signal because the m33 was removing that from the mains signal.
Your idea to run a rum signal from an oscilloscope to amp is good. If you have a normal DAC or anything anywhere in the house,.running it straight to the amps is good.
