Display MoreLet's make this straight:
There are 3 Volumes.
#1 Master Volume - you control this with the Master Volume knob on the unit; it has no effect on the tone; changes the unit's output level.
#2 Amp Volume - you control this with the Volume knob on the unit; it has no effect on the tone; changes the current rig's volume.
#3 Cab Volume - you control this in the Cabinet section; it has no effect on the tone; changes the current rig's volume.
The thing you are talking about is achieveable by the Power Tube Sag parameter in the Amp's settings.
By the way, although ckemper has now confirmed that none of these volumes alter the sound, I have to agree with guitarnet that I also feel the difference between using the Amp or Cab Volume, even in tone - somehow using the Cab Volume makes the tone fatter and more responsive for me.
No, I was talking about Master Volume of the amp section. My real Road King sounds different the louder you turn it up, better actually. I believe power sagging reduces power supply voltage which results in a change dynamics/compression.