The DD12 plus is an amazing sub, especially as it comes with a built in SMS-1 (bass parametric EQ to tune the sub to the nth degree).
I auditioned one against the SVS's I have and thought the DD12 better and more musical.

It came down to price - I wanted twin subs and 2 x DD12's were a lot more then The SVS's as the SVS's were 2nd hand and the Velodynes new.

I don't know what you have in your 861 , but I run my subs off the analog L/R sub outputs with my 568.2, you would need some sort of analog output for the Velo.

The Velo's do have strong bass management built in, if you had to go for a DSW sub, I would imagine you would have to use something like MRC instead to get the best out of it.

Roon/Tidal, SBT, Trinnov ST2 , Twin Devialet D premiers, Vivid Audio Giya G1 Spirits, dedicated and treated room.
Meridian gear: 568.2mm and DSP5000's acting as TV speakers.