I have been wanting to try M/S processing and some stereo to mono crossover processing in my next mix.
I think the easiest experiment to try would be to split the master with a crossover, send the lows to a track that is converted to mono. Personally I would start with 60 Hz and mess around with the crossover point. Like RedDog said, if you have a surround sound setup things that were recorded in phase when played in the surround sound setup could easily cancel out due to the fact that there is still a lot of low end energy in the satellites. Summing up all the low end from both left and right and sending it to a sub or center will be more compatible with all range of systems from 5.1, 2.1, stereo and mono.
For stereo and mono setups it could be easy to do basic M/S processing. On the master create a low shelf that removes lows on the side channel and then inversely boost the same frequency with another shelf on the mid channel. The end result is that there will be a smooth transition from the lows going form stereo to mono, the lower the frequency, the more mono it will be, or in other words, the stereo width will be less as the frequency goes down and will be at mono where the shelf is at maximum. +/- 20dB shelf should be plenty to effectively make the bottom end effectively mono, hell even +/- 6 or 12 dB would work, you would have to experiment and also experiment with the crossover point (q of the shelf, and the resonant frequency etc.)
