I heard the Hertz ML2500 sub on a few occasions in Ryan's 135i, and it was phenomenal. The sub played extremely low for a 10" and it had a very clean sound. In his car, it was easy to get the sub perfectly balanced with the front stage - completely transparent, but turn it up and crank the dub step. Hit this link for a post showing it mounted in the corner of the trunk.
JL makes fantastic subs as well, and I think you would be happy with their products. Price....well, depends on where you get them.