Musical Subwoofer for 2-channel system

I would try and up your budge to $2k, and look into a REL B3 for a truly musical sub.

At $700 you are in the Vandersteen 2Wq range, they are know to be very musical definitely worth checking out.

Based on my experience with three different subwoofers from them, I recommend you listen to Mirage before you buy anything. Their subs are musical first and foremost, and are easy to blend. Furthermore, with the room size you have, you want a small one.

I specifically recommend the Mirage Prestige 8 sub. 8" driver in a 12" cube. Crossover adjustable from 40 to 120 Hz. Linear down into the 20s. Uses an 8" fiberglass composite cone with elliptical surround, very similar to the bass drivers in their $7500/pair flagship OMD-28 full-range speakers. Price of the Prestige 8 is smack dab in your price range at $579. For deeper bass and more output, move up to the Prestige 10 at $750.

I can tell you, those elliptical surrounds WORK! They allow the driver to make bigger excursions and stay linear, getting more articulate, deeper bass than you would otherwise get for a given size, loading, and power. I have a pair of their OMD-15 floorstanders, and with those elliptical surrounds, the bass quality and quantity produced by their 5.5" drivers is a big surprise. You really hear what *kind* of bass you're listening to, be it mic'd acoustic, contact pickup acoustic, electric, flatwound or roundwound strings, which pickup was selected, etc.

Lets throw in there the MJ Acoustic subwoofers. I currently use there 100 MKII in my living room with seamless integration thanks to it wireless remote. Plus it has a variable phase adjustment making set-up more of a breeze. The most musical sub IMO under a grand.