Does a Subwoofer give relief to my mains speakers?

My main speakers have two 12" woofers per cabinet and give great bass response for movies....though sometimes when the bass gets really low and powerful on dvd's (Gladiator battlefield scene), it causes the woofers to sound like they are coming apart....this never happens with music. Will adding a subwoofer relieve the stress on the woofers in my main speakers? Will the bass be as powerful? I'm considering the Velodyne HGS 18 or the Infinity HPS 1000. My room is 22ft wide and 70 ft long with 22 ft high ceilings....carpeted concrete floors and concrete ceiling....the building was once a beer brewery and has been converted to live-in lofts.

Preamp..............Audio Research LS-10 solid state (no phono input)
Preamp..............Carver C-19 Tube (I have it connected to the AR LS-10 and use it for a phono preamp)
Main Speakers............Infinity Kappa 9 (bi-amped)
Amp.................Carver Silver 9-t monoblocks (low pass)
Amp.................Sunfire Signature (high pass)
Rear Speakers.......Infinity Kappa 7
Center Speaker......Infinity Kappa Series II center
Amp (rears).........Carver TFM 42
Amp (center)........Carver TFM 42
Phonograph..........Oracle Delphi Mk-I
Cartridge...........Grado Reference Platinum
CD Player...........Carver SDA/360 5 disc changer
Tuner...............Sansui TU-X1
Surround Processor....Technics SH-AC5000