I went through this process last year as can be seen in my thread:http://www.axiomaudio.com/boards/ubbthre...ewal#Post380021
My room is open style, measuring 3900 cu ft with hardwood floors that is larger than yours if you have 8' ceilings (3200 cu ft). The prevailing wisdom here was that the M2s & EP400 would not be adequate for a room of that size. Well that has proven not to be the case so don't underestimate the performance of this little sub. My system is unquestionably up to the task for 'music only', but I would agree not for 'HT' unless there are 2 or more 400s.
I listen to Electric Blues, Fusion Jazz, Latin Jazz, Big Band, some Classical (including pipe organ) & to a lessening degree, Rock. With my EP400 set at about the 1/2 Vol Level, I've measured about 85 - 90 Db in my near-field listening area that is about 8' from my sub. Never does my system seem strained at those levels. In fact the EP400 easily thumps my couch with authority; however, if you often listen at Disco/Party type levels at the far end of the room, then more likely a 12 inch unit or 2 would be the way to go.
Just one happy user's opinion, FWIW...