Yes, the Elliptrac 400. The M200 has a very low compression ratio, which supposedly gives it a more "relaxed" sound than other compression drivers. (It's basically a 2 inch diaphragm feeding a 2 inch exit, without a phase plug.) It would seem like a natural choice for the Elliptrac, but it has an upper limit of 4 KHz and therefore needs a tweeter.
If I understood your post correctly, you have the Elliptrac and the HP640s? I've seen the HP640s in your other posts, but not the Elliptrac. I have a pair of EV HR90s sitting on top of my Belles, but they have 1.4 inch throats and will not fit the M200.