The MartinLogan subs are, IMHO, better than velodyne. They are tighter, and have much better transient response.
But, my advice to you is wait til you get them home before you decide to purchase a sub. You might find that their bass is satisfactory all on its own, like me.
"the question is do you want a sub?" not necessarily. but i am putting a 43"plasma tv in this room and i do not immediatly plan to do full blown 5.1 system. i will use the ml's for front speakers when watching a dvd. but they are mainly for music.
if they are mainly for music, they "SHOULD" give you enough bass. I auditioned them with all kinds of music, from Sade, Diana Krall, Sarah Brightman, Josh Groban, James Taylor, The Rippingtons, Spyro Gyra, Shirley Horn, Cher, Madonna, and Keiko Matsui.
As any of you Keiko Matsui fans know, her music is LOADED with VERY deep bass. While the Clarity didn't hit the really deep notes, they certainly put some vibrations in my chest.
I would try them without a sub first. If you find you need one, take a look at the ML line. Their least expensive is around $600, and from the reviews I have read, is outstanding.