I think that at that price point you are rite on the line between going with something in the $200-250 range {or 2 of them} and the VTF2 mk4 {I own 2 of} for a little under $600... So its up to you, I love the vtf2 its the all around best value for your money, its loud, low, and clean, not huge, but sounds huge.. my room is a little bigger than your and one turns it into a theater, 2 cracked the ceiling...
if you are interested in dual subs of less cost, then either 2 bics or 2 polk 505's will also sound good, having 2 more than doubles the output and makes rooms easier to tame... but your budget is on the line of 2 vs 1 vtf2... kind of hard call to make... 1 sub will work a lot harder than 2 subs for the same sound but the vtf2 has a great sound, I never had the chance to put 2 505s vs 2 vtf 2 side by side but have heard them both and both will do fine for your room....