Definitely the R-1. Specifically for your rig, too.
Here's why:
660- Too big to stealth IMHO, and too noisy without the ACM mod. But once the mod is done, you lose line-ine. Which means that your Core Sound Stealths wouldn't work without an 1/8inch to xlr converter. And even then, the price is high.
MT- The main POSSIBLY salvageable virtue that's been discussed so far, amdist all the bugs and design issues, is that it MAY record digi-in via spdif accurately, should the firmware pan out. But this is useless for anyone not running a pre or A/D convertor that has digi-out via spdif. Difference between the high-end pros looking for the MT to be a storage device ONLY, and the rest of us who would've hoped to actually use it for the A/D and preamp. In other words, if your CS stealths are going into a battery box and then into the MT, you would need an adapter for your 1/8th inch to two 1/4 trs plugs, which is where you are put specifically into the exact issues which are currently wrong with this unit, since you would actually be trying to use it as the "all-in-one" it was supposed to be. Throw in the power issues and unit freezing and really, what else could possibly be wrong with this unit?
R-1- It's small, it records in 24/44.1. It's remarkably easy to use and it uses 2 AA batteries. Has line-in via 1/8inch input, and has more manual features. Starts up quick, sounds great. Even if you upgraded your mics and got a pre, you would still be able to run this unit with excellent results via line-in.