I know exactly where you're coming from bro. Unfortunately, the only real solution I've found was running 2 heads and a switcher. Like Glassman said, it's not so much your preamp, as it is the EL34's in the power section. Once I switched my Tremoverb over to EL34's it sounds freaking awesome, but it did lose that tight low end grunt and definition when you turn the gain up or switch modes. I think that's the main reason I still the keep the Triple Rect around(well that and all the hassle I had to go through to get it lol).
In all honesty, if you're looking for that heavier Mesa type tone, the only place you're going to get it is a rectifier series amp. If it were me, I'd look for a used single rectifier and go from there.
Then again, the Mark V and the RoadKing/Roadster amps would work just as well. I know you said you didn't think you would use the 4 channels on the Roadster, but ya never know. I don't know what the prices are up there, but here, the used prices on the MarkV and the Roadster are pretty close to each other.