I think you're getting into an area that is discussed fairly thoroughly on the Amptone webpages.

What you heard on MTV was mostly pre-amp distortion, and because of the high gain in the input stage, everything sensed by the pickups came out loud. If Slash had used a lower-power tube amp, the power stage could have been driven into the 'sweet spot' without having to rely on a high-gain preamp stage for distortion. This is the problem that got Master Volume type amps such a bad reputation.

I did some work earlier this year on a 5-Watt tube amp I called the WeeBeastie. It sounds great, but I still have to shake out some bugs so that I can maintain the tone I want while adjusting the power level.