Eldartford, what you were doing with the center channel is not "bridging". In bridging (also called monoblocking, monobridging or summing) a stereo amp becomes a single-channel (mono) amp with much higher power (usually about 3X the rated power).
The setup you described with the third speaker is called a Hafler surround circuit. In a Hafler circuit, the third speaker is fed a left-minus-right signal. Anything that appears equally in both channels is cancelled, and what is left contains a lot of out-of-phase ambient information.
The Hafler circuit was used in the early surround-sound Quad systems with LP. I used one for quite a few years for movie sound, and it sounds fantastic with certain types of music. The Audio Research surround sound system is actually a Hafler circuit. There's a reviewer at The Absolute Sound who uses a Hafler setup for all his listening. NAD gives directions with their stereo amps for implementing a Hafler circuit, but it can be done with any stereo amp. All you need to do is connect the third speaker across the positive terminals of the stereo amp and connect to the positive and negative terminals of the speakers. Properly implemented, the ambient channel should be run at 10dB below the mains.
I've often thought I should go back to a Hafler system for movies. It has some advantages over Dolby and DTS.
