Subwoofer for Vienna Acoustics Mozart Grand

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.

I found this article on subs which was very interesting.
http://www.soundoctor.com/whitepapers/subs.htm#2SUBS
check it out, goes into great detail about correct set-up, placement, crossover etc. well worth a read.

The only problem is now you KNOW that you need (must have) TWO subs AND an active crossover!!