[zero the hero]

RS180s are hardly midwoofers. Why not use something cleaner in the range you're going to use them, like the RS150's? The 180s have a nasty cone breakup that you can't deal with using an active XO. Also, 3 270s is way too low impedence, even for the A500. You also realize that 3 RS270s need about 9 cubic feet sealed, if you're going for a Qtc of .7? That's a big speaker!

 

[zero the hero]

Don't get me wrong - the RS150 has nasty breakups too just like the RS180. But if you're only using them as mids you are definately better off with the 150 - better off-axis response and some people complain of poor midrange in the 180.
2500 is too high though, esp for an MTM. I wouldnt go higher than 2100 for the RS150, 1500 or so for the 180.

Oh, and I don't have any driver suggestions for the woofer. I really like the RS270 myself and would love to build a speaker around a pair based on its low distortion and awesome sensitivity (at the price of needing a huge box - tradeoffs)