Best subwoofer for 2 channel Hi End system

I need your suggest because I want to add a subwoofer to my Hi End system.
I have a pair of Tannoy Edinburgh HE loudspeakers with a Pass Aleph amplifier. Althought the bass are very good I want to add a subwoofer to reproduce very low freq.
Please let me know your opinion and recomendations.
I have Avantgarde Uno's with an Aleph 3. I use a Talon Roc sub to help with the bass. Very musical and very fast. You can buy one used on Audiogon for less than $1500. At that price point, it would be hard to beat.
Because I believe that even very low frequencies provide directional information, and because most subs are front firing, I've never accepted the "one-subwoofer-is-all-you-need" theory. And down-firing subs, although theoretically omnidirectional, always seemed to lack good transient response (which is very important to me since my mains are electrostats.)

Recently, I installed a single new Martin Logan Depth, for reasons which I explain in the comments to my system specs (under my ID "nsgarch") here on AgoN.
Vandersteen 2WQ. Even better, a pair of Vandersteen 2WQs.
Vandersteen 2WQ, Ditto.
I have a VMPS Original Sub with all the extra's possible. Its hits a low 19hz or lower. It can make the house shake with no noise as its below the human listening level in some movies. These are not self powered so they will require you to feed power to them from an outboard source. Some buy plate amps and attach them to the sub box. I instead went with a Rotel amp to power mine. A pair of these would be plenty of low bass in my opinion. Go with the VMPS Larger sub to go even lower. Beware though, these subs are large in size, so check dimensions before buying!
I totally agree with Swampwalker. The 2Wq is an excellent sub for music and with the "Q" adjustment, you can get it to integrate seemlessly with about any speaker.
A pair is better for several reasons. One, it loads the room better for smoother response and second, there is some information in the lower range that is channel dependent.
I personally have found that 2 enhances the soundstage width and depth over using 1 sub that sums the low frequency information.
Also, as a final thought, the 2Wq is driven at speaker level. This removes the responsibility from the preamp and also allows the sub to take on the general characteristics of the driving amplifier for better blending.
It is a well designed and executed sub.
IMO, you'd be well advised to determine HOW to do it well & IF it's worth doing it. The "best" subwoof can be chosen based on the above.
At that (excellent) level of spkr you'll prefer a stereo (i.e. two subwoofs). BUT as your spkr officially specs -6db @ ~30 Hz AND there's probably some room gain too, all you'd be doing is adding energy btween 35-20Hz... that last (less than one) octave. However, if you insist:), here's one way of doing it:
* hi-pass the Tannoys at ~40-50Hz with a line level filter (i.e. before the amp).
* narrow down woof choices, referring to their linearity in the lowest octave (15-30 or 20-40). EBS alignment woofs may be the ticket here.
* your speakers are very good & IMO need subwoofs usually described as "fast" (it's a description of transient, flat q, matching with the main spkrs, etc; there's no such thing, really).
* maybe dipole subwoofs (the german "attac 400" come to mind) or, if unavailable, closed boxes -- so that the sound doesn't seem to lag or result into what's called "bass heavy", lagging.

Finally, of the few commercial designs I've heard, I liked the big Velodyne, the big A-Physic and a Vandersteen model. I heard all these in stereo mode (i.e. two), placed next to the main spkrs.
I have tried many high end subs before and now I own the new Velodyne Digital Drive DD-15 and DD-18 for my music room. You will only need one of these subs.

You can use the included kit to see how the sub reacts to your room and adjust it accordingly. I can't believe the difference and improvement once you had done this.

Nick T
I use a Velodyne HGS-15 with a pair of bi-wired KEF 104/2s. A Proceed processor sends everything below 80 Hz to the sub, which results in even more open and transparent sound, even though the KEFs are capable down into the 20 Hz range. Freeing stereo speakers of low frequency duty seems beneficial. (I use a Proceed Amp-3, and each channel can deliver 250 watts into the 4 ohm load of the efficient KEFs.) The KEF to Velodyne transition is seamless. I plan to add another HGS-15 on general principle, even though LFE is smooth and strong throughout room with a single HGS-15.
Aerial sw 12 hands down
1) Aerial SW 12
2) Revel B15
3) ML Descent
4) The other stuff
I listened to everything, and wanted the Aerial, but found the $1.5k premium over the Revel to be too much. So I ended up with two B15's.
If you think two is too much, then you don't know about bass nodes, or have a perfect room with lots of acoustical treatments.
Main speakers - ML SL-3's.
amp they use now. Very musical and an extremely powerful capability. No servo's needed here as distortion is quite low already. At 140 lbs quite heavy. VMPS has been getting great reviews on their bass since the eighties. Bass is typically articulate with natural weight but could be used for home theater too. For the money they are one of THE bargains in subs. One of the best benefits though is the advantage of being able to adjust bass "Q" to your room.
I would suggest the green mountain audio hammer. Better yet, two green mountain audio hammers.
I met Roy of Green mountain at the show and he is a hands on kind of guy. I have always had much respect for his work but after meeting him , the respect carries to the man as well.
I just picked up a Martin Logan Depth for my Aeris I. What a great Improvement over the Vel HGS-10.
Nick T is right on.
No other sub is as adaptable to one's room and system.