I had a 400+ for 5 years. It's a GREAT head. However, I didn't care for it at extremely loud volumes. It's break-up just gets too grindy/overdriven, and not in a good way, like Ampeg, for instance. Some people love that, but for me, it was sweetest at medium to medium-loud volumes. I traded it for a Mesa Titan which, to me, sounds great at insane volumes, but probably not a sweet or rich as the 400+ at medium volumes. The Big Block's are great doing what they do. Getting a tubey sound with the quickness of solid-state. The built in OD/distortion works very well too.
I've only played the Carbine's in the store, and they too are great at what they do. More of a hi-fi/clean sound at any volume. It's all personal preference, really. The M6/212 is a great combo, but I agree that ANY 212 cab will have a difficult time in a loud metal situation.