I just purchased a Martin Logan Depth for my 2-channel system recently. I chose the Depth after an exhausting search for the "right", musical subwoofer. I listened to subs from Velodyne, JM Lab, REL, Titan, VMPS, and Martin Logan's smaller Grotto. They were all very good, but I had to give the nod to the Depth.
The Depth is exceptionally fast, taut, clear, defined, controlled, and blends with my JM Lab monitors seamlessly. I really wanted to spend only around $1,200 for a sub, but I absolutely felt compelled to get the $1,700 Depth. I also heard the $2,700 Descent, and it was better, but not necessarily in terms of clarity; it just had more physical impact.
Good luck!