Is size a restraint, or are you looking at output/$? If you are looking for smaller (comparatively) sealed units, and want duals for around $3k, the SB13-Ultras and the F15HP's from Rythmik are both pretty well reviewed, and would probably have more output than the Velo.
Duals offer several advantages over a single sub when it comes to smoothing frequency response, especially over multiple seats. That alone may be enough reason to sell the Velo, the added headroom would then just be an additional plus.
If size is not an issue, ported subs generally will give you more output/$ and several other options are available in the same price range (Dual SVS PB12+, dual Rythmik FV15(HP), dual HSU VTF-15, dual Power Sound Audio XV-30's etc).