Best subwoofer for 2 channel Hi End system

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

hi
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.