I just went through this myself (this morning actually) when trying to pair an older Earthquake subwoofer with the EP-600. It actually was quite a bit easier than I expected to produce a flat response in my listening position and to a lesser extent throughout the room. I could get the same relatively flat response in my primary listening position with just the 600, but adding the Earthquake really helped in the non-primary seating positions as well. Without the second sub, drop offs in other positions were very noticeable.
I used the Behringer Feedback Destroyer (BFD) equalizer to smooth the response once the subs were positioned... it is an awesome little tool that I just cannot say enough about (of course, I found out about it here on the Axiom forum). It has a bypass button on it that allows you to turn off the processing and compare results with and without it... it's an incredible difference. If you've got a well-treated room the BFD may be overkill for your room, but in my case, it was a great addition to the home theater.
