One of the great things about subwoofers is...
...that, unlike other speakers in your system, timbre matching is not a concern. Between the options available on most HT receivers, and adjustments on the subwoofer itself, pretty much any subwoofer can be seamlesly blended into any system. You aren't forced to work within your main speakers family.
Depending on where you are located, some subs may be available to you that aren'y available elsewhere and likewise, some subs available to me may not be available to you at reasonable prices due to shipping, duties, etc...
I'd suggest you look into SVS or Hsu but then again, the might not be available where you are at the same prices I can get them for.
so, if you want some meaningful input, specifying where you are might help.