Please help with subwoofer.

I'm considering one new Velodyne HGS 18 Series II with a 1200 watt amp ....or two Velodyne F-1800 with 600 watt amps. 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. Will the one Velodyne HGS 18 do the trick or will the two less powerful F-1800's be better. The cost for one new HGS 18 is the same as two used F-1800's...$2000. I've never owned a sub before and I'd appreciate your input....thanks.....Mitch

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
Television............Sharp 32 inch