At the risk of repeating what others have said, I've owned an SVT 810 cab, and I currently own a pair of B212s.
In terms of tone, they are polar opposites. The former is like a lo-mid flamethrower. The latter has a much more ambitious reach to the bottom and sounds relatively polite, and there's also a response hole in the upper mids before the tweeter

sets to work.
Second, I'd hesitate putting an SVT head -- or any head of that class for that matter -- on top of only one B212 cab. I think you'd be asking for trouble in terms of driver damage. Forget about driver ratings for a moment. The B212 is a reflex cab that I suspect will unload its drivers at a frequency where the SVT head is probably still capable of delivering full output. I would offer the same caution even with two B212 cabs. This is far less apt to occur with the sealed 810. A subsonic filter in the effects loop is one way around the problem. But even apart from damage due to subsonics, an SVT into a pair of twelves just doesn't pass my Best Practices test unless you are really careful/attentive and never, ever let anyone else play through it (which you shouldn't do anyway, regardless of your kit, but that's a whole 'nother thread).