I'll chime in here too. The Mod50 has an incredible power section. In my experience, with all the modules I had with it, it felt and sounded like the Mod50 power section was built to complement modules like the SL and Bman, and the like. Mainly the more Marshall type flavors. When I moved to the M4, it took me getting a really nice power amp (VHT) to get the same kind of vibe. I've not owned the RT 2/50, but since moving to the rack based setup, I've not been happier with my tone. I definitely don't feel like I'm missing anything, not having an actual head. I think in the end, the differences are small enough that it boils down to: do you want 2 modules or 4 modules. Can't go wrong either way.