velodyne dd plus? still considered best of the best?

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.  

Hardly relevant anymore...David Hall, incredibly smart guy, invented LIDAR and sold it to the US Military.  Velodyne is a shell of what it once was and is barely in business.  Because of LIDAR, David certainly doesn't need to live on subs anymore.

Please take a look at the Paradigm Defiance Series Subs.  Start at $400 and go to $1499. Starting at $549 they have Anthem Room Correction which can be run on an Apple or Windows PC or a Mobile Device,  Full App Control of the sub from a mobile device; which allows you to find all the rattles in your room at specific frequencies by doing a "Sweep" (from 20Hz - 120Hz that you can lock and adjust in 1Hz increments) so you can isolate those things rattling at every frequency and know what frequency it is.  It also allows you to hear where the dips are in the room at those frequencies.  And finally, all those have wireless capability as well if you can't get a physical wire run where you need the sub to be. 

X-Series made in Canada from $999 - $1499 and V-Series made in China from $399-$649.  X-Series woofers have two US Patents = 1) Increases output by +3dB, so double the output, and 2) Reduces the distortion by 50%, so cuts that in half...Musical and Dynamic for movies!

 https://www.paradigm.com/en/overview/series/defiance-series   

Velodyne has not made a sub in a long time. they where good in their time, best in home theater. Good customer support, but not sure if they repair them any more. They have moved on from the sub business. I was at their factory 5-7  years ago and the sub was just a very small area of the plant and becoming smaller.  I had several of their subs with my last being HGS 18 for my home theater. Great sub. but i sold it a few years ago and have moved on to 2 channel only system. You should check if they are still servicing the subs before buying one.

How about this:  https://destinationhifi.com/products/legacy-goliath-xd-subwoofer?variant=336275489

Of course not