The power stage of the amp is the very last section. It's only function is to boost the signal from the preamp to drive the speaker(s). All of the tone shaping is done in the preamp, which is where the gain and EQ is applied to the signal from the guitar.

Channel Volume @ 10; Master Volume @ 1:

The preamp is getting all of the guitar signal. Some of the preamp tubes will be overdriven, producing compression and distortion. The power amp will receive a small, distorted signal from the preamp. There will be no power amp distortion. The sound level from the speakers will be quite low due to the Master setting.


Channel Volume @ 1: Master Volume @10:

The preamp will receive only a small amount of the guitar signal. Unless a lot of gain is applied, this signal will be passed along to the power amp as a 'clean' signal. The high setting of the Master Volume will allow all of the preamp signal to reach the power amp, but because it is fairly small, it will probably not be enough to cause the power amp to distort. The result is likely to be a clean sound from the speakers.


Neither of these situatons is likely to produce power amp distortion. That will only happen if more signal from the preamp is fed to the power amp.

In the first example, raising the Master Volume setting will introduce more of the distorted preamp signal to the power amp. Since this signal is already distorted, the sound from the speakers will be very distorted. Think Metal.

In the second example, raising the Channel Volume will pass more of the guitar signal to the preamp, which will then have more to give to the power amp. Assuming that the preamp does not introduce distortion of it's own, a large clean signal will reach the power amp. If this is strong enough to saturate the power tubes, the result will be a sweet, touch-sensitive distortion. Classic Rock & Blues.

As Lee The V used to say, "Your mileage may vary".
:)