First check these video's, for those who never heard of it the digitech cabdryvr is an IR cab. solution pedal.
So they don't use an amp...I mentioned before as currently I'm using an old processor as a clean amp, it has a super basic cab. sim. but gives me 5 bands of parametric eq. With those eq's I replicate clean preamps either by ear or for example, I make a preset using amplitube or any vsti then analyze its responce and replicate that with the eq.
This isn't do-able for many guitar players out there but products hint evolution to that direction. Amp-in-a-box pedals are already popular, even wampler makes an amp for them now. Everything is getting more and more customizeable.
Imagine; drive from a vox, eq from a fender, cab from marshall, poweramp sound from mesa...won't take too long until someone makes a processor that works in this fashion but easy enough for a guitar player to use. It's already happening in the plug-in world and even analog to some extent.