Dealing with a large room like yours, you need powerful amp and/or high efficiency speakers, a pair of subwoofers will help to relieve the receiver and main speakers.
http://www.audioholics.com/home-theater-connection/common-mistakes
Inappropriate gear for the venueHeavyweight speakers and amps might overpower a small room, but I don’t necessarily consider that a bad thing. Overkill is generally good, whether it be wattage, horsepower, pizza, or Nutella. What is inappropriate, however, is a low-powered system with low-sensitivity speakers in a large room. And you can exacerbate that problem by not including sufficient subwoofage. Most often, speakers with a sensitivity rating of 86 dB @ 1 meter will not work well in a 5,000 cubic foot room, regardless of the size of the amplifier. In a larger room, you may have to step up from a receiver to separates, and move to speakers that handle more power and have higher sensitivity. And if your 300-watt 10” sub makes rude flatulent noises during loud passages, it isn’t necessarily the sub’s fault. If you have under-spec’ed a subwoofer for a room, you can compensate somewhat by placing it in a corner where it will couple better and have more output, but in some cases may sound boomy in a corner. Consider buying multiple subs. Not only will the output be cleaner because each driver and amp is being driven less hard for a given system SPL, but you can place them strategically in the room to smooth out the room modes. If two are placed laterally on opposite midwalls, they can minimize the null to improve bass in the center of the room where most people place their seats.