Subwoofer size for 1500 cubic foot room

Considering purchasing a Subwoofer. After reviewing and testing several different set of speakers in my listening room (local dealers allowed me to bring them home) I felt that the improvement over my old Thiels is not warranted. However, temporarily using my surround sound systems sub(from our family room) has definitely produced a difference. I am now on the hunt for a sub. Looking at 8-10" models priced up to $1250.00. Velodyne seems to have a great deal on the Optimum 8 (for $699.00 reg $999.00) my only hesitation is whether or not 8" is sufficient for a 12x14x9 room. I listen to music at moderate to moderately loud levels and watch movies with this system, as well. Any comments on this sub and its size.
I owned the Optimum 8 and used in a 13x15x8' room. You don't need any more.

You might also consider the new Rel T7. I just switched to that and found, while not going quite as deep, it has a much nicer upper/midbass and integrates better. It vitally disappears, which I could not get the Velodyne to do.

Both are very good subs for the money and all you need in your room.
one of the small HSU Research would be fine.
Yes I have the same sized room and one Velodyne dd10 The sub plays nicely with my Omegas but I am looking for a mate. Not for more bass but for stereo and yeah I know you cant hear bass direction below this or that frequency. I`m too tired for a debate.
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.
If you read my other thread on whether or not to replace or refurbish my Thiels. I just found a great pair of new speakers the Monitor Audio RX6 for $1250/pair. Brought them home on a trial. They were much better then my Thiels with a significantly lower extension in the bass area. Sufficient to postpone my desire for a Sub. Thanks for your recommendations.