Musical Subwoofer for 2-channel system

I am looking augment bass to my 2-channel system. My system consists of:

Bryston 3BSST Power Amplifier (170W RMS/channel)
VTL 2.5 Pre-amplifier
Cambridge Audio 840C V2 CD Player
Custom 2-Way Monitors with Accuton Ceramic Drivers

I am looking for a musical subwoofer around $700. My room is 15x15x10 (LxWxH).
Will a passive subwoofer like the SVS Passive Cylinder 25-31 CS–Plus do a decent job? I am also open to active subs.
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.
A used Rel Strata fits your criteria: very musical, easily blended and low and high inputs.
I've narrowed my search to either 2 JL Audio F113 Fathoms or two Velodyne DD 18s. I still haven't decided. Can anyone offer some help?

I excited about the JL Audio for all the great press I've seen lately. I've noticed it is used in several members systems.

The Velodyne's setup routine sounds like it is superior to all others.

The Velodynes' also has a servo mechanism, I'm wondering how effective it is and if this makes it superior to all other subs?

Again I will only be using this in a two channel system for music only.
Please help me decide.
It would be great if anyone knows how to contact Larry Greenhill from Stereophile? He has reviewed and reportedly owned both subs, so I'd love for him to respond.

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