Thanks again for the great reply, Mark.
Good to know I should get a matching system - that clarifies things a lot despite making the decision in some ways harder.
I've checked just about everwhere for availability of the Fours, and it's just not looking good (or even possible) until at least November. Apparently from what I'm told the demand for them is crazy and NHT is putting their focus on newer models.
After a lot of thought (and I may still see if I can listen but that's iffy), I think bottom line I'm going to try the Mythos ST series. I spoke with Definitive techs, and if configured right, yes, the center is two-way, but with proper AVR setup, the lower end of the bass can be channeled to the ST's. The rest of the Ten is apparently an ST without the subwoofer, which should sound pretty great.
I did also give Paradigm (Monitor series) a look, but I'd again have to go with a separate subwoofer here, which I'd rather avoid if possible.
The bottom line is my room isn't huge, and the Mythos ST series will fit what I'm trying to do far better than anything out there at the moment (even the NHT's, though I might have gone with those if they had been available). It also requires less complexity for someone new to high-end home theater. I realize I won't have quite as much control over the bass as with something like the X2, but for me, that might be a boon, and the receiver I'm getting (probably the Pioneer Elite SC-37) does a pretty good job, from what I've read, of figuring that out.
To give some perspective, this is what I'm upgrading from: http://www.crutchfield.com/S-lqvwJFb...=detailed_info
It's not exactly what I'd consider a minor upgrade to be sure! ;o) Still open to all feedback til I order (hopefully towards the end of this week!).