I'd start with a single Ultra if that is what you are thinking. It can handle a pretty large area. One of the guys here has a fairly open floorplan and it still absolutely pounds you and still sounds clean.
Since they don't have the Servo15 anymore, I would say go with dual Seismics instead of stepping up to the 15 since it is technically a lower model and only shows a -3dB rating of 35Hz, vs the 17Hz of the Seismic.