Active Subwoofer Recommendations

At that stated $500 price-point, there is minimal to nil audio performance differentiations between your contenders and pretenders. It’s largely a “just pick on of ‘em” exercise for you.

Personally, I would strongly recommend that you stretch the budget and go with a pre-owned quality build B&W PV1 home cinema subwoofer, as the ideal synergy match to your sound bar.

google the reviews.

A pair of small 8" units, sealed, might work quite well. you don't need much, it may seem loud, but it is still just a sound bar.
I've no idea how the B&W is wired for handling a sub, though.

Rythmik L12 is <$550. It should be good unless your space is huge.

+1 on the Rythmik. Servo Subs.....yum

Paradigm Defiance V10 Retail $550 - Has Anthem Room Correction and full App Control from your phone - wireless capable although doing that is a last resort, better to run a wire - check it out at Paradigm.com and look up videos on YouTube about the Defiance Subs.