Well, it won't be *instead* of the pre-amp, it will be *with* the pre-amp. If you run the pre-amp lower and cleaner, say with the Volume 1 at 5 or so, you won't get enough signal to clip the power section until the Master is cranked. That will be very loud, which I assume you're trying to avoid.
A low volumes, you can't tell the difference anyway. If you insist on clipping the power section, and wearing out the power tubes faster, try an attenuator, which plugs in between the amp and the speaker (not the effects loop). The sound and EQ will change, and there are lots of opinions on whether the changes are good or bad.
Some say the Master is too touchy, but use the Lead Master to fine tune the loudness level. Dial in the level of grind you like, then use the Master and Lead Master to set the loudness. EQ to taste, bearing in mind that EQ at very low volumes is very different than EQ when playing loud. More treble and bass at low levels should do the trick. Or cut mids, it's the same thing.
Easy peasy. No need to over-think it.