There's a new flagship from REL – the R-528 Special Edition. This new subwoofer improves on the 528 thanks to a handful of upgrades and a smaller overall footprint. You'll find similar tech to what's in their higher end Gibraltar G-1 system.
The folks at SVS have introduced a pair of new subs that may not blow your mind with their feature set, but will certainly impress with their low price point. At $499 each, the SB-1000 and PB-1000 both sport solid specs for the cost.