Best subwoofer for 2 channel Hi End system

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.

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!