My room isn't much bigger, 14x20x8.5 and I do feel the room benefited quite a bit from 2 subs. My primary sub is the EP-600 and the secondary is also a 12" sub, but it is in a box easily 1/4 as small as the 600. The FR of the second sub also isn't nearly as low as the 600, nor does it play as loud, however, I feel it has helped to smooth response in the room.
This is more noticeable when sitting in spots other than the primary listening position, so it is _not_ something I notice in most situations. The 600 sounds just fine to be by itself when I'm sitting in my main listening position (as I positioned the 600 to sound best there). So in my case the added volume wasn't the main reason for putting the 2nd sub in, it was more for smoothed response. If I hadn't already owned the smaller sub I probably would not have gone out and purchased a second sub for my room, but given that I had it already, I felt it was better to include it in the setup than not.
