Subwoofer for Vienna Acoustics Mozart Grand

REL has often been recommended with Viennas as well as Sonus Fabers. Maybe because of the Sumiko connection over the years but regardless, I believe there are many satisfied owners using this combination.
I use a REL T1 with my VA Bach Grands and couldn't be happier. The T1 is a fast, musical sub that syncs with my Bach Grands very well.

Sounds like good advise! Thank you

You need to do some research. I suggest beginning with the DSpeaker processor which has the ability to make almost any sub work with your room and speakers.

In an in home comparison REL were a worst performer. Compared to the other subwoofers their connectivity and setup instructions were simply silly when I tried their Studio III.

I disagree one of the most musical subwoofers I have heard was a REL, the one I heard was very well setup though. I also like the Rhythmik/GR research servo 12. Most of the challenge with a subwoofer though is going to be integration, so I definetly recommend something like the mini DSP or Behringer 2496. Most of the bad subs I have heard in my life have been very poorly setup or way too hot.

The Velodyne is very good as well, most of its strength though comes from its auto setup mic, most smaller audiophile companies can't afford to develop something like that. I heard a huge Velodyne DD sub sound very good in a tiny room.

Make sure you try & check out JL Audio as well.
Less room optimisation than the Velodyne but much better value for money IMO.
I wouldn't be to focused on the room optimisation features of the sub itself. While helpful if you want proper integration you'll really want to look at an active crossover like the Behringer 2496 mentioned above or the Bryston 10B-Sub if budget allows for it.
But all that being said you probably cant go wrong with any of the three brands mentioned. Just keep it out of the corner if possible & dont turn it up too loud, less is more.