I too have long subscribed to the multiple sub theory, but two X13's were just too much for my theater. Fully treated room is only 2100ft3, and i could not back these monsters off enough so they would not dominate the room. At one point during an explosive action scene using DD Master 7.2, i thought maybe i just made the room bigger !
Im sending one of them back - one is plenty for me !
I will however augment the X13 with one of my smaller subs because i just have to be able to say ive got two !!
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Double drywalled, "Green Glue" treated Theater
GIK acoustic treatment
Epson 8700UB Projecting on a 135" Seymour XD Audio transparent screen
Outlaw 976 Processor
Outlaw 755 amp for Center and all four surrounds
Adcom GFA 555II for Main L & R channels
Behringer EP4000 for two homemade subs
Outlaw Ultra X13 for REAL Sub Bass !
OPPO BDP103, Marantz CD63SE, Denon DP300F
2 Adcom ACE515s for AC current treatment
Newport Theater Director for AC distribution.
Speakers are all built by myself utilizing Dynaudio Drivers
Second room: Yamaha receiver, Musical fidelity X10D tubes, Carver C9 holographic pro.