Do you need full-range speakers with great subs?

If you've got a couple of great subwoofers, do you need full-range speakers for the front and surrounds in a really big room? I currently have full-range floor standing speakers for every channel. The fronts are 6 ft tall with two 12 inch woofers per speaker and the rears are 5 ft tall with two 12 inch woofers per speaker. I'd like to reduce the size of the fronts and surrounds to large bookshelf sized speakers, but I don't want to compromise the sound quality. I'm currently running 5.1 and I'd like to go to 7.1 to take full advantage of the new HD sound formats.

I'm using one sub with 18 inc woofer.

My room is 70 ft long by 22 ft wide with a 22 ft ceiling. A pretty big room for a residence, but I'd rather not have 8 monoliths in the room. It is also my living room and not a dedicated theater.

The center speaker is 25 ft from my sitting position, the fronts are about 30 feet away and the surrounds are 25 feet behind me in a 5.1 set up. Because of seating, I cannot put the surrounds directly to the sides of the seating position as a lot of seating diagrams recommend.

When I go to movie theaters, the speakers along side the wall overhead don't seem that large, but there are a lot of them though. I think I counted about sixteen total speakers along side the walls and in the back.

Below are the specs of the speakers that I am considering for fronts, backs, and surrounds in a 7.1 setup.

Infinity Kappa 6

Specifications:

Cabinet dimensions: 25'' H x 15'' W x 10.5'' D.
Power handling: 150 Watts RMS.
Nominal Impedance: 4-6 ohms.
Frequency Response: 39Hz to 45 kHz.
Crossover Frequencies: 800 Hz and 4.5 kHz.
Emit Tweeters.
3" Polypropylene dome midrange
10" Injection Molded Graphite Woofer

I welcome your comments and suggestions.

mitch