Best subwoofer for 2 channel Hi End system

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.

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.

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!

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.
Cheers