I have 3 XV15se in a 2400^3 room with one smallish sized opening to the rest of the house. They are reference capable down to 16-18hz. I think duals placed near field would do ok in your room but would not be reference capable below 25-30hz in your room. The nice thing is you can try them free for 30days. If they are not enough then simply return them.
For example: using data from the ULF thread based on a 5000^3 room.
Dual XV15se would net max output levels around 113.5db @ 20hz and 103db @ 16hz.
Dual VTF15.2 1 port would net max output levels around 112db @ 20hz, 108db @ 16hz, and 101db @ 12hz.
Dual FV15HP 1 port would net max output levels of 114db @ 20hz, 110db @ 16hz, 104.5db @ 12hz.
The point is, none of these options will get you reference level output below 20hz. The Rythmik and HSU will have a big advantage in the 12-16hz range, but again not reference capable. SO if you are chasing big output in the ULF region it might be time to look at Captivator 2400's or build your own ported 18's. I think the goal should be to have a reference capable system in the 16-80hz region if you are going to invest the money.
Dual Cap 2400's 15hz tune would net max output levels of 118db @ 20 & 16hz, and 108db @ 12hz. Now were talking! Big rooms need big subs.
