Having built the MTM Version of the double 10 i would probably recommend the MMT version until a more finalised MTM version is available. It's an easier build and you won't have to deal with the issues of the horn cavity creating reflections. Obviously it will be a taller speaker but most of the time you want that tweeter nice and high!
As for 10 vs 12, you will want to stick with the 10" drivers for this type of design as the spacing is perfect for a 650Hz crossover. If you want to use 12's then I would recommend Pete's PM60/90 with the HF950/ND950 combo if you're trying to reduce costs. Harder design to build but higher output.
As for the 950 combo, I've been running it with a 24db/oct crossover. It's sounding great! As has been mentioned before, the ND950 is used in several of RCF's own speakers crossing at 650hz so I am more than confident that the tweeter is happy at that crossover.
Not had the chance to let my build fully rip yet but theoretically I should be looking at 132+ dB from my MTM mid tops.
