I'm just asking a general question. I just changed all the tubes in my Soldano & I believe the preamp tubes seemed to affect the tone more & the power tubes affected the VOLUME (it made it twice as loud) & the break up. They also added a bit different dynamic & now artificial harmonics ring better.
Short answer: Both. Seriously. In your case, if your SLO is trimmed out to run fairly clean in the power section, then yes the preamp has more impact on amp coloration simply because it's running in less-linear fashion.
However, when you twist Volume vs. Master Volume, what you're really doing is controlling for how you blend preamp and power amp distortion. (In the much broader sense of that term, btw.) Incidentally, IMHO, that sort of blending is an immensely powerful tool for shaping the tone and character of your rig, and MV is specifically designed to give you that power. "Volume" is only secondarily about volume control.
But if you want a better answer...and some time to ruminate...not for the faint of heart...click here.