Ahh, the Muse 18. I will better you, the Muse 16, which looked like a large, wood refrigerator on its side. I heard one at Shelley's Stereo with Magnpan 3.3s in the mid 1990s. Amazingly articulate, tuneful bass, until it bottomed.
Going back to the OP's question. I would pass on the DD for several reasons. 1) Velodyne has all but abandon subwoofers and long term support is questionable. 2) There are better alternatives now. 3) The alternatives are better for the money.
I used Velodynes for years, ULDs, F series, SPL, and HGS. I have since moved onto Rythmik. The Rythmiks offer a tighter, clearer, drier sound, while also having greater output capability. In my office, I use an L12, which I say is slightly cleaner sounding than the HGS-12. In my main system, a F15HP, which was a significant upgrade from the HGS it replaced in sound quality and output.
For the same money as a used DD-15 (about $1400), you can get an F15HP, or enjoy the benefits of two subs (smoothing of room modes) with two F12-300s ($700 each on Rythmik's clearance page). You will also get a warranty.
Finally, for music, you may want to also look at the HSU ULS-15 MK2 dual drive (two subs) for $1499 a pair.