Between a dual VTF3 and a ULS 15, I would go with a dual VTF3. One on one the ULS is a bit tighter and cleaner, but two VTF3s will give you endless clean headroom at 20 hz and probably two or three times the deep bass around the tuning point. Just compare the CEA measurements at Hsu's site. Not to mention the room smoothing effects of duals. The thing is, the VTF15h might be a big sub, but the VTF3 isn't really that much smaller. It's about the size of an endtable. Two VTF3s in a 2300'^3 room will give you pretty hard bass and remain very clean to high levels.