You can get the Outlaw ICBM bass management unit which will allow you to set all your speakers to large and then set the cross over frequency for each set of speakers (Fronts, Center, Side Surrounds, Back Surround) it will also permit stereo sub pre-outs. Works very, very well.
Another problem with soundstage in a HT configuration is that the speakers are usually behind the plane of the TV screen. Try moving your fronts so they are closer to the seating position than the screen.
What is your source and receiver/processor/amp set up? Also, what are the room dimensions?
