Can any multi-channel amp be bridged?

When you bridge a stereo amp, you drive each individual amp with the same signal, but one signal has inverted phase. Note that this makes for a more steady draw from the plus and minus power supplies. Some amps (like the Dynaco ST120 of long ago) only meet their specified power rating, both channels driven, when the signals are out of phase. A bridged power amp is operating in a "balanced" configuration. If that's good for preamps, why not for power amps?

I have not used amps in "bridged" mode, but I did do something very similar for many years. I reversed the wires of one channel at the phono pickup, with corresponding reversal of the speaker wires, and then bridged my stereo amp with a center channel speaker. Since most of the signal even in a stereo record, is common mode (monaural, horizontal groove modulation) what I was doing was very close to what today is called bridging. The results were much better than I had any right to expect. Perhaps it was because phase coherency of the three-speaker set was guaranteed by all being driven by the same amp. No amp ever complained about this setup.

So, my conclusion is that, although all the downside comments seem reasonable, bridging can work well. I am sure that 8 ohm speakers are preferable to 4 ohms, but frankly, I didn't have a problem even with 4 ohms. Go figure.

Audiobomber...Also called the Dynaco matrix multichannel setup, and they sold hardware to facilitate its setup. They derived a center front channel by using a Y connection of the front Left and Right with the return through the center. What I did was to make the fronts the back by reversing the phase of one stereo channel. If I played a monaural source, my setup would be exactly the same as "bridging". The center was driven by the differential amp outputs, with the two front speakers driven by the single ended outputs. I preferred my setup to the Dynaco Y-connected center because I don't like the idea of putting speakers in series.
Other advantages were easy control of the center channel volume (all it takes is a "blend" pot), and easy derivation of a low level "rear" signal, Left plus Right, (plus because one signal is inverted) so that the rears can be driven with a separate amp. The only difficulty was getting the phase inversion for other sources, like FM Tuner.

Audiobomber...Of course the required center speaker signal is L+R, but there is more than one way to make this happen. One way is to mix the low level L and R signals, and drive a separate amp. Another way is to make a Y connection of the three front speakers, so that the returns from L and R are tied together and then connected to the High terminal of the center speaker, whose Low goes to the Amp "common". My way was to invert the phase of one channel (eg: Right) and then to bridge the amp with the center speaker. In this case the signal that the center speaker receives is (L-(-R)), which of course is L+R.

Think about it some more and draw a picture (again I wish that the Audiogon site had some way to include a diagram in a post). My setup actually resulted in a lower impedance load on the amp than mono bridging, because in addition to the center speaker across the two amps, each amp was also loaded with a speaker, L or R. But, somehow I never had a problem.

Back in the very early days of matrix multichannel I did a lot of experimentation, and discovered lots of interesting things that never got incorporated in commercially marketed equipment. I even created my own logic-assisted system before these were on the market, using a DBX expander/compresser. Most of the problems with logic-assisted matrix systems can be overcome if the process is digitally implemented.