Budget ?
You didn't say how much your budget is, but two Polk Audio PSW505 subwoofers only cost about $500. $500 is a pretty low budget for such a large room. I would recommend a good SVS sub, but even what I would consider minimum for that size room, a SVS PB12 Plus, is $1199. Two of those would do wonders or one SVS PB13- Ultra $1,799
A really good low cost sub is the Dayton Sub-120 HT for only $160. Four (4) of these, one in each corner would cost only $640 and would sound very good.
If you get more than one subwoofer, you best bet no matter what you choose, is to get two exactly alike.
Forest Man