Thanks! I love the MBFusion with them. It's legit tube old school to my ears. It sounds very different than the MB200; the tube preamp really imparts a tube sound, complete with squish and fullness. It also has tube headroom, meaning it's difficult to get the gain very high without getting into tube distortion territory (fine with me, I love a little tube hair). I think this is why people tend to recommend the 800 since you can run out of clean headroom quickly. Because of this I've yet to feel worried about the 500w aspect and pushing these speakers. I don't play with heavy distortion much, but I imagine the tube pre will compress before you get too much into the damage type clipping. It does have big GK low end, but I always run an fdeck HPF for a little security, anyway (just as I did with the MB200). Speaking of, anyone running these cabs should use an fdeck or microthumpinator, it really lets them roar.
That said, it does have more oomph than the MB200 for sure. I love the sound of the 210 and 15 cab. It's more well rounded than the double 210s, more even sounding because it balances the coloration of the sealed 210 a bit. I prefer that sealed sound and of course the portability of the double 210s, but I always bring the 15 for outdoors gigs, and the Fusion pushes this combo better than the MB200 did. I recently had a new matching grill made by Bruce at Fliptops: