velodyne dd plus? still considered best of the best?

The DD and DD+ can be an incredible value in the used market considering their performance and features. Servo motor with manual control, auto as well as manual room correction, visualizing frequency response, low distortion, very well built, massive amp, sealed, remote, etc.
The DDs probably have the smallest cabinets in their respective woofer sizes of any premium sub. The DD+ versions have bigger cabinets but lovely aesthetics with the curved sides and esp the black glossy veneer. 

The DD subs use to be cutting edge, but like others I would not purchase one now with the new ownership and questionable support.

One of the most musical subs I've heard is the Aerial Acoustics SW-12, however these are pricier than the others you mentioned.

I would go with the SVS sealed and not look back.

Good luck.

They're not considered the best of the best but they're good enough.  Deep bass doesn't need to be perfect.  The room causes far more problems than the driver.  I've got a 14 year old DD18 that works fine and I'd never consider replacing it unless it stopped working and I couldn't get it fixed.  It's a great product.  The deepest bass without a hint of audible distortion.  Sealed box means no port noise.  There's auto eq and manual eq.  It's a thoroughly thought out product with several generations before it.  The question is just whether you want to risk buying an excellent older product that might have a part fail.  Can you get a replacement?  

Despite its age the DD Plus simply offers more than everything on your list. Its condition and how it was used could be an issue.  

The DD Plus is a sea change from HGS and a substantial improvement over the DD.
 
The JL Audio F series are as potent but their DSP is Auto only so your stuck with JL's choice of how a sub should sound. The version 2 added more frequency bands but it remains Auto only.
JL Audio now offers the CR-1 crossover for some manual settings of a single setup.

You would need to contact Audio Reference and ask about service support https://www.velodyneacoustics.com/
George Meyer AV in Los Angeles CA. was an authorized Velodyne service center https://www.georgemeyer-av.com

Reading the SVS Ultra manual it may be the closest to the Velodyne Auto/Manual concept. It offers three EQ presets and Q adjustability but not having any practical experience with it I can't comment.

I posted in another thread that Velodyne has sold its sub line to another company, so hopefully we’ll see some service available soon. I love my pair of DD10 plus, though the remote sensor seems to be less responsive lately. The nice thing is not only does it have high level speaker inputs like the REL, but you can also hook up the xlr or rca. I use high level for stereo and xlr for Home theater system, so subs work for both my systems.

The ddplus was designed for low bass in 20’s and below, and I like having ability to change crossover slope via remote.

They keep up well with my TAD’s which have wonderful bass, but roll off in low 30’s.   I like very tight bass with no overshoot, but the servo can be lowered for looser bass.

I’m using them in a 12,000 cubic foot space, and though I don’t like a lot of bass, they do satisfy my needs.