My first thought is that the MTM is a great choice and hasn't gotten enough attention, but there's plenty of reasons to go with other designs.
I like the MTM because if its improved sensitivity over the single midwoofer version, but that comes with lower impedance. That's probably not a problem, but something to be aware of. Of course, the two midwoofers should help with evening out the in-room response over the two rows of your theater, so that's one of the biggest plusses, IMO. Obviously, the same voicing in the regular alchemy can be had for a good deal less money. WIth both these designs, you get the EOS waveguide which doesn't quite meet the theoretical goals of the SEOS, so that's a point against them, I suppose, but a small one. Also, and check with someone who knows better, but I think Jeff uses BSC in these designs, while some of the others may not.
The Pure save money over the MTM and gets you the SEOS over the EOS, has lots of sensitivity and is an easy driving 8 ohms. Lot's of good there. Again, I'm not sure about BSC.
The Tempest, on the other hand is a great design, IMO, but you are paying a lot more and you won't make use of the low extension. If I were going to splurge and extend the budget, I'd go for Tuxedocivic's 1099.
Of course, I've not heard any of these designs. I have a set of Cheap Thrills (bwaslo's design and SEOS waveguide - designer of the Karma-10 as well, which is lower priced than most of the designs you mentioned) that I'm very happy with. Of course, the list of folks who have been anything short of very happy with any of DIYsoundgroup's offerings is very short (maybe one or two, and that's questionable).