vijay: While Def Tech makes some nice speakers, their specifications are simply bogus, and I've never understood why they do this. For instance, the SM-55 which I auditioned a year or so ago is stated by Def Tech to go down to 32 Hz. Huh? Simply a lie. While it has very good bass for its size, a well-known publication measured it 3 dB down at 44 Hz (which is very good indeed, but a far cry from 32 Hz). So what do you think that means for the Promonitor series or Def Tech subs? Try to find some reviews on the model you're looking at that actually give test results. And most importantly, audition them. And when you are auditioning make sure you bring a dB meter when comparing different speakers. Speakers that have approximately the same frequency range can sound very different. There are many factors in why (better drivers, lower distortion, better dispersion characteristics, and on and on).
Bookshelf speakers, even those that have a decent to good bass response, will be greatly enhanced by a good subwoofer. It's really a requirement. Even full-range towers can benefit greatly. When using a bookshelf speaker that requires a crossover above 80 Hz, localization will come into play, meaning your sub will need to be placed close to your mains. With bookshelf speakers that can go low without audible distortion, it allows you to place the subwoofer anywhere in the room because the crossover point will be under 80 Hz (the subwoofer crawl test will locate the sub for you).
As far as the different models by those I.D. companies, it depends upon what you are willing to spend. While I.D. companies will give you far more bang for the buck than their commercial counterparts (and better warranty), in a room (or rooms) that have your volume, one needs a subwoofer with significant output and possibly multiple subs. Give me a price range and I'll offer up a few suggestions to think about. Better yet, go to the subwoofer section of this forum and you'll get plenty of knowledgable people helping you.
I wish you the best on your search. It should be an exciting (and hopefully), rewarding experience.
Last edited by dsrussell; 07-12-2014 at 02:07 PM.