Ultimate sub?

Check out SVS,they make real nice subs.

The new revel ultima looks killer

Of commercial subs I haven't heard anything to beat the Martin Logan Depth I - I have one myself.

That's commercial subs - I have found speaker manufactures, if you contact them, often can make you one specifically for whatever speaker you have. By this I mean if say your speaker is ported it would be 24 db per octave at whatever your speaker goes down to - similar if its sealed and you would use 12db. This matching makes a big difference in my experience and I will be getting new ones specially tailored for a new speaker I have on order.

Thanks
Bill

Velodyne DD+ review in the March issue of Stereophile discusses and compares a few other subwoofers which may be of interest.

If you decide on a unit without digital signal processing/EQ DSpeaker processors seem to be well received.

Personally, I'm on my third generation of Velodyne now running two DD12+ and using the manual setup procedure. More than anything the ability to be able to adjust presets, phase, and volume, on the fly from the listening position makes the sub experience fun.

Best I have heard was at a Krell demo,the sub was the Master Reference.