The Buster is generally considered Mesa's cleanest tube amp. I'm a 400+ player, and there are definitely times I wish it had a little more clean headroom (and I did the phase inverter preamp tube swap). I do find that having quality power is important for keeping the 400+ running in the clean. Less than optimal voltage situations really cause the amp to breakup a lot sooner.
As far as the hybrid debate, I actually prefer the opposite of most hybrids. I like a solid-state frontend and a tube power section. I'm not a big fan of a clipped frontend, but I like a little hair on the output. I really dig the sound of my Bmax through my Mesa's FX loop. If they weren't so rare nowadays, I'd like to find an Aguilar DB728 and pair it with a solid-state preamp.
When it comes to the tubes vs. solid-state debate, I'm squarely in the middle. There are things I love about tube amps that just haven't been present with the solid-state gear I've used (e.g., low-end warmth, the three-dimensional and physical presence of the notes), and there are things I love about solid-state gear that just haven't been present in the tube gear I've used (e.g., quickness, extreme clarity, upper-harmonic presence).
It's great that as bass players we have the choice between tubes and transistors, which both sound great. And you can get a great sounding solid-state amp at a relatively affordable price. For guitarists, to get a good sounding solid-state amp (e.g., Axe-Fx), you have to pay about as much as a professional-level tube amp.