Need some help picking a subwoofer

REL Storm is basically a Strata with a more powerful amp, slightly larger cabinet (same footprint), and an interface with slightly different input options. It is a fine woofer, too; one replaced my Strata II.

Nola Thunderbolt II is deep, defined, musical and relatively affordable.

I don't have any experience with the above subs, but my JamesEMB1500 is absolutely fabulous in my system. Very tight and fast and extremely musical. Prodigious bass, but the sub is particulary wonderful at low volumes for Jazz trio music and the like. That 4 string bass is wonderful. Rich, deep with the gentle feel of moving air. You have not mentioned your room size, but I would go with two if you have the $$, and bigger is better in subland. There is a JamesEMB1500 for sale right now on the 'gon for a killer price. Not "midpriced" new, but maybe in your ballpark, used. Sterophile did a very impressive review of the 1200 that you can find in their archives, FWIW. warren :)

Try the Revel Sub 30. This thing is superb with the correct setup. Speaking of which, in my experience unless you have someone who can use instruments to set up the sub with your system you'll never get out of it what you paid for it. But once so calibrated look out!