10" subwoofer around $500, used or new?

Go REL or Sunfire.

Thanks Bob, the level should be variable (I assume you mean volume will impact the sub volume). And yeah no crossover in the SLI80, so the L5 will be full range. 

BTW are you affiliated with SVS?

Oh also, I may end up placing it on another system: Neat Iota & Bel Canto DAC 1.7 & REF S150. Iota high base is really weak, low base does not exist. Currently using a Velodyne DPS 12. Would the SVS offer a noticeable improvement?

Why settle for a 10" when for $539 shipped you can get a Rythmik L12, with not only a 12" woofer, but a servo-feedback controlled one?!

I’m very happy with a pair of SVS SB1000. A sealed design with cabinet under 14x14x14". The driver is 12 inch. As Bob Reynolds mentioned, the SB1000 allows for speaker level or line level hook up. It also has continuously variable adjustments for crossover, phase & gain. SVS offers a 45 day in-home trial with free shipping on returns. So, while I have no idea how it would compare to your Velodyne, you could decide for yourself. 5 year warranty too.

And, NO, I’m not affiliated with SVS other than having been a paying customer; nor am I being coerced, induced or otherwise motivated by them to reply to you.

Sure. Lots of options out there. You can definitely spend more and might even get something audibly better as a result.