Personally, I have dual XV-15's in and 1800 cu ft. room. 15hz at 115db (measured) is pretty good.
I also have dual (I like sealed subs, and also have dual Epik Legends, 2 12" sealed woofers with only
a 300W amp). They kick butt in the mid bass area, and go down to 19hz in a stereo only
listening room I have.
I can't imagine what dual 15" XS-30's would be like. I would of got those but they were not out yet when I ordered the XV-15's. Tom felt that dual XV-15's would be best given the size of my
room, and what I wanted to achieve.
But XS-30's really look like an Empire on steroids....at an unbeatable price. Dual opposed
15" woofers, 750w Bash amp, weighing in at 111 lbs. Whats not to like? Being sealed they will sound tight. My little old Legends, with just 2 12" driver with a puny 300w amp are great. Especially with mid bass slam.
Dual opposed 15" woofers is a great design, and the XS-30 will hit 9hz to 14hz in a typical
listening room ?.....that's more than nice.