Doadman, you have accurately identified that boosting a driven amp and providing a distorted signal to a clean amp require very different pedals to get a nice, tight modern high gain sound.
I have rather more experience in this area into a clean amp, where the Distortus Maximus is very good. Even better, though is the Digitech Hardwire Valve Distortion. Each distortion pedal itself can be treated like a "mini-amp" and can be boosted by an OD pedal (often a TS or variant) in front of it. This often works very well at getting awesome high gain tones.
The problem with doing that is noise. The more gain stages you use, the more noise you will generate. So, unless you have noise suppression very well sorted, you will end up with intolerable levels of noise.
Unfortunately the same can easily be true when boosting dirty amps with OD pedals, so if you are going to go that route, then a noise gate of some sort may well be necessary, if you don't have one already.
If you are looking for a booster pedal for a dirty amp, then consider an MI Audio Blue Boy Deluxe, which I found suited all sorts of different amps and circuits. The beauty of the BBD is that is doesn't have the traditional mid hump of a tube screamer (unless you set it like that !). Thoroughly recommended.
