The MC-12 is considerably higher priced than the others you mention. It's a fabulous piece of gear, IMO. It's absolutely killer on HT, with the Logic-7 decoding being the best on the market, both in my opinion and in many others'. For two-channel sound it's a big step up from the MC-1 though, again, I'm not sure that the differences you hear would be worth the money to a non-audiophile.
While many focus on just the sonic qualities (which are clearly most important), there are other factors that make the Lexicon a great buy. Lexicon products have always been very user friendly, while still being very tweaky if you decide you want to get more adventurous. Lexicon has always had a great trade-in program, which is the only form of true infinite upgrade-ability. The MC-12 is a brand new architecture, and has a load of capability yet to be incorporated, but very possible given the architecture.
I get a bit tired of all the opinions saying brand X is crap, you should buy brand Y. I love Krell gear, but to come away with the notion that Krell outperforms the MC-12 (or even the MC-1) on HT surround would just be unfortunate - review after review speaks of how the Lexicon product line (Logic 7) is the best decoding algorithm on the market. That the Krell outperforms the Lex in 2-channel mode is possible, given any given person's preferences, but to deduce that the Lex is an underperfomer in this area would be wrong. If you're interested in multi-channel synthesis from two-channel sources (many people are) there's no better than the MC-12 (with Meridian being the only real competition). If you like any of the other models (and there are many, many reasons to like the other models mentioned) more than the MC-12, great, but the MC-12 is a tremendous product.
Finally, The Perfect Vision has a review of the MC-12 by Anthony Cordesman in this month's issue. It would be worth getting a copy if you get serious about buying one. -Kirk