How It Works
Opamp U1A is connected as a subtracting amplifier. Should the same signal be applied to both inputs, the output is zero. As a result, it will remove all common information from the stereo signal, and reproduce only the difference signal - in exactly the same way as the original Hafler design.
U1B is a simple summing amplifier, and the output contains all the information from both the left and right channels. A possibility that springs to mind is that we could then subtract the difference information from this output, so that only material that is absolutely common to both channels would be reproduced.
Centre Channel Control
The pot (VR1) is to set the centre channel level. This can be a trimpot, or a conventional pot mounted at the rear (to help prevent 'fiddlers' from mucking up the settings you like). I have seen circuits which do not include this, which seems basically a bad idea. When the two channels are passively summed, the centre channel will typically have a level of -3dB relative to the left and right channels - provided the signal is not mono. Centre channel speech (for example) will be mono, so the level will be equal to that of each of the main speakers. Since the centre channel amp and speakers are rarely as powerful as the main Left and Right channels, there is a distinct possibility of overload of the amp, the speaker or both.
Since the centre channel is supposed only to fill the 'hole' and provide a stable centre sound image, it does not need to be as loud - especially since it will almost certainly have inferior sound quality to the main speakers and will therefore degrade the overall sound quality. The level control will allow you to set the level to just sufficient to provide a stable sound image, and no more. In my system, I do not use a centre channel, and doing so would have an overall adverse effect on sound quality. If you have good main speakers and have a stable and well defined soundstage, a centre speaker is likely to do more harm than good.
The low pass filter using R7 and C1 is optional. It provides a nominal 8kHz roll-off frequency (which is apparently quite normal for 'real' surround-sound processors). This helps to minimise any disturbance to the main stereo signal, but feel free to leave it out, since most rear channel speakers probably won't be able to reproduce much above this frequency anyway. If you do leave out C1, R7 should be omitted and replaced by a link. Note that the rear speakers must be wired to their power amps in reverse phase - the Left channel would connect normally (red to red, black to black), and the Right channel is wired in reverse (red to black, black to red).
The sub-woofer output is simply taken directly from the centre channel mixer, and no low-pass filter is included because I don't know of any sub which does not have a filter already. Adding another one simply adds unnecessary complexity, and will introduce phase shift at the output that a phase compensation circuit (often included in sub woofers) may not be able to cope with.
The 100 Ohm resistors in the outputs are to prevent the capacitance of the signal leads causing the opamps to oscillate. At this value, they will cause no high frequency loss, unless you insist on 100m long signal leads (in my experience, these are uncommon).
It will also be noticed that there are two outputs for the rear speakers, simply in parallel. I included this because it is easier to wire if the user is connecting a stereo amp for the rear speakers. Naturally, a mono amp will do just fine, as long as it is capable of driving the two rear speakers in parallel (but out of phase with each other). This may not be possible if the speakers are 4 Ohm types (these are becoming more common in hi-fi, so its not that silly). If you do have 4 ohm speakers, you can connect them in series. To obtain the out-of-phase connection, join the red terminals, and connect the two black speaker terminals to the amplifier's output.