10" subwoofer around $500, used or new?

My recommendation is Gallo TR 1D 595.00 new.  It is a sealed design and is much smaller than any of the box design subs. 

" Since you say it's primarily for music, I suggest you consider sealed subs. It would also help a great deal if you stated what speakers and electronics the sub will be used with "

Currently using JBL L5s and Cary Audio SLI-80. I may not stay with this setup though, so a sub that is not to picky (is that a thing?) would be a better choice.

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.

I’ve owned this with Spendors. You can’t go wrong with SVS!
https://www.svsound.com/products/sb-1000

Check out Speedwoofer.. They have an excellent sub @ $399.00 shipped that is incredible. I purchased one for my Magnepan .7's and was more than impressed. I plan on purchasing a second. Their customer service is outstanding, I talked to the owner on the phone for set up advice. Don't let the low price fool you..

My fabulous (!) REL Q150e was bought for 200 bucks (Ebay) a few years ago and except for a missing grill cloth (easy fix) it was perfect. A great sub. Made my own Neutrik Speakon plug cable from some good wire and saved some bucks there also (love Speakon plugs by the way).