After 2 years of working with subs, my conclusion is:
Just make sure that you have digital room correction for the sub(s).
This will address the issue you're trying to fix. Beyond that, any good sub - there are lots - will probably do the trick. However, do take a look at the subwoofer tests at HTshack.com. Make sure that there's enough LF extension, clean output capability, and damping (sometimes called "speed" on these threads, measured as "group delay" on the test pages) for your taste. Disclaimer: Some out there equate "speed" with small drivers, Group Delay has no apparent correllation to driver size. Make your own call on this one.
I use 2 Rythmik subs, but have auditioned JL extensively (terrific, but pricey). Both measure very well (Rythmiks better group delay - JL more clean output capability way down low). Many of the SVS models measure almost as well as JL (the SVS cabinets are much bigger and less "sexy") at a fraction of the price. The JL's include a basic DRC function, the Rythmiks and SVS require an external DRC unit. Mine is the Velodyne SMS-1 (I also use an NHT x-1 x-over for low cut on the main speakers, because I'm not crazy about the internal low cut circuit included in the Velo SMS-1). SVS sells a DRC box they designed with Audyssey - it requires an external x-over if you choose to low cut your mains.
Good Luck
Just make sure that you have digital room correction for the sub(s).
This will address the issue you're trying to fix. Beyond that, any good sub - there are lots - will probably do the trick. However, do take a look at the subwoofer tests at HTshack.com. Make sure that there's enough LF extension, clean output capability, and damping (sometimes called "speed" on these threads, measured as "group delay" on the test pages) for your taste. Disclaimer: Some out there equate "speed" with small drivers, Group Delay has no apparent correllation to driver size. Make your own call on this one.
I use 2 Rythmik subs, but have auditioned JL extensively (terrific, but pricey). Both measure very well (Rythmiks better group delay - JL more clean output capability way down low). Many of the SVS models measure almost as well as JL (the SVS cabinets are much bigger and less "sexy") at a fraction of the price. The JL's include a basic DRC function, the Rythmiks and SVS require an external DRC unit. Mine is the Velodyne SMS-1 (I also use an NHT x-1 x-over for low cut on the main speakers, because I'm not crazy about the internal low cut circuit included in the Velo SMS-1). SVS sells a DRC box they designed with Audyssey - it requires an external x-over if you choose to low cut your mains.
Good Luck