need a good or the right subwoofer for my set up (1 Viewer)

hello, all, I'm newb on this form and my first post, I'm in the finishing stages of my basement to be completed and build a home theater , only thing is i cant decide what subwoofer to get, My room is very open 15 ft X 34 ft long and 9ft ceilings, i have all the wires and cables roughed in , and set up for 7.1 system, i have harmon Kardon AVR 2600 and KEF slim T05 speaker,this set up will be strictly used to watch movie and nothing else, and my budget is under $900, since i have a have a huge open room i need something that will fill up the room, Ive been told i should buy a 15" because of the open area, but i find the 15's to be very rattly and slow ? please let me know what u guys recommend ? so far the one that i think that's i heard and sounds good was Klipsch 12' SW112B or Velodyne subs, pls let me know what you guys recommend,thanks

i find the 15's to be very rattly and slow ?

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Which 15" home theater subs have you auditioned that have this "rattly" issue? What do you mean by "slow"? Because Dan Wiggins and Dr. David Hyre proved that woofers traveled at the same speed no matter the size - http://www.adireaudio.com/Files/WooferSpeed.pdf

so far the one that i think that's i heard and sounds good was Klipsch 12' SW112B or Velodyne subs

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If you think a Klipsch sub is good then you will really like these: http://hsuresearch.com/products/vtf-15h.html http://www.edesignaudio.com/product_info.php?cPath=2_41&products_id=618 http://www.epiksubwoofers.com/empire.html If you build it yourself, you can easily go with dual 15's.