SVS, ED, Hsu, Outlaw Audio, Epik, etc. all make excellent subs that generally outperform 'most' brand name loudspeaker equivalents for less money.
Of course, when money is less of an issue, then REL, Revel, higher end Velodyne, and of course JLs come to the forefront.
Sorry if I left some out. Considering there are some 100 reputable speaker manufacturers there are bound to be more than a dozen sub companies.