I haven't heard the Velo but personally, I would order the 500 to see what you may be missing. I would find out if it is possible to over-drive the Velo. Just because it's DSP-controlled doesn't mean that it can't be over-driven. The 500 on the other hand cannot be over-driven.

The Velo will most likely lack 3 Hz or so on the low end. This may or may not be that important to you.

$150 is a heck of deal for a sub like this!

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated