velodyne dd plus? still considered best of the best?

So, did you ever decide on a sub?

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.