GCG, just a couple of general points first...
Rather than room volume, the more applicable metric to speaker capability is the distance from the speaker to the listening position. Do you also have longer than average listening distance (i.e. greater than say 12ft)? If you have a broad listening area, wide uniform dispersion is also desirable, which is relevant to your need a large soundstage.
Conversely, the bass frequencies produced by subs "see" the total room volume and a proportion of any adjacent open areas. So the larger the room (and adjacent open areas), the more capable the sub(s) must be to produce a satisfying bass performance.
Having said all that, and assuming longer than average listening distance, a very good option for a large format bookshelf/monitor speaker within your budget is a pair of
Ascend Acoustics CMT-340 SE's. These are higher than average sensitivity with very wide dispersion and the published measurements are excellent. At about $600 shipped they're great value and will allow you to put a bit more towards the sub(s).