Nick, I am in the same situation as Sean in that I have what would appear on paper and to the average Axiomite the absolute mismatch made in hades, a first generation EP350 and an EP600 coupled together along opposite mid-sidewalls of a 14' x 25' dedicated room that was brought together by pure happenstance. I've ran this setup for a couple of years maybe and without throwing numbers around that I probably wouldn't know too much about anyway I can tell you that the combination works surprisingly very well.
I am soon to part ways with that little EP350 so just maybe I'll finally pair the 600 with another same or very similar sub upon much procrastination after six years.
My vote would be a pair of matching subs to fill that gaps that a single might not be able to accomplish on it's own but as always and depending on your particular room, layout and acoustics your mileage may very.
