velodyne dd plus? still considered best of the best?

I almost went with a pair of the Rythmik 12" subs.  But decided to go with the REL S3's because of their high level connection.  They blend quite well because when connected that way they act like an additional woofer to help extend the bass without it sounding too boomy.  However, I really like the direct servo technology uses in their Rythmik subwoofers.  However, since they are sold direct it was difficult to buy them without hearing them first.  I have heard they are very musical.

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?