Wow great thread! Subs aren't always as straight forward a choice as other things. Watts, cabinet design, driver implementation, ported, bandpass, sealed, frequency range, SPL output, efficiency. There is a lot that goes into finding the right sub for a particular room.
For me the choice was a bit simpler, I wanted to stay with Martin Logan if at all possible. The Descent was the natural choice, its design is a perfect match for my ML speakers. It plays low, offers very clean yet powerful bass, with it being a sealed design using three matched subs in a triangular arrangement, there is no cabinet vibration (I tried the coin thing and it's true).
But I still have another sub in my setup that I just love and have no plans on getting rid of any time soon. That sub is an A/D/S MS3. It's not nearly as expensive as the ML Descent which is $3k, the MS3 was $1,300 when I got it 14 years ago. But the design of the of the A/D/S is unique, it's a sealed design on the inside and ported design on the outside. It's very efficient, outputs very high SPL's, has in room response down to 20Hz, and has not ever distorted or over driven, even at very high volumes. And what's interesting is that it uses a sole 10 inch high excursion driver, and in my room it out performed a more expensive Paradigm sub and a 15 inch Earthquake sub with over 1,000 watts of power. The Earthquake was loud, and did play low, but was a bit too boomy, and not nearly as quick and tight as the A/D/S.
So smaller subs with less power can indeed match or surpass a larger sub with more power. But it all depends on the whole design. From the time I got my MS3 14 years ago till now it's been a flawless performer, and the only negative I can say about it, is that it's no longer being made.
So I can rightfully say Pirate, that one doesn't need to spend thousands on a sub to get amazing performance. And also the sub doesn't need to be gargantuan in size with 1,000 watts+ of power to kick ass. Heck my MS3 only has a 150 watt amp and it delivers the kind of output of a 12-15 incher that has a lot more power.
Finding the right sub is tricky, and there are so many to choose from in all kinds of varieties. So I'd say if it's at all possible, try some out in your room and see how they sound for you.
Seth