Need some help picking a subwoofer

I too have the concertos and use a SF Gravis as my sub. Can sound a bit wooly unless the signal is very clean - other than that, very good.

I've read that REL Strata III mates seamlessly with the concertino and the concerto, like it does with most speakers period. They are available used for about $800-1000, but are sold very quikly when they appear on the used market. Good Luck.

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 :)