I have to put in here for the Mirage mm8. They list at $800 each but you could easily get two from Vann's (which is an authorized dealer with return privilege and full factory warranty) for $700. This is well within your budget, and if you're not too confident about a single 8" sub filling your room, I'm sure you're OK with two such subs, especially if they're each packing about 300w rms/1200w peak.
I just added a pair of these to a much larger listening area (open architecture living space with vaulted ceiling to augment Mirage OMD-15 floorstanders) and they do the job very nicely. They are not subterranean in extension; about the lowest they'll add is 32-36 Hz if they're set against the walls.
But they have a lot going for them especially for music systems. They are tiny--a 9" cube. This is small enough to tuck two of them into a 9x12 room. They practically disappear. They are extremely quick and blend with mains or small satellites easily, and they have the proper controls for getting a good blend--volume, crossover (from 50-200 Hz or bypass), and especially, 0-360 deg. continuous phase control. They are very punchy and quick. They may not give you subterranean explosions, but they flesh out the bottom end of bass viol, bass guitar, tympani, etc., and are lively and quick for adding punch and rhythm from kick drum.
A funny thing since adding them--and I still run my mains full range--everything sounds more dimensional now. The various voices have more space around them and present a more 3D space, and the overall presentation is even more timbrally correct (once I properly dialed in level match, crossover, and phase) and musically involving. I'm enjoying my 2-channel system for music more than ever before.
As an alternative with lower reach and more dynamic range, if you want a single sub to double for music and video, and be sure to fill your 1500 cu. ft. space, look into the
SVS NS12SD.
