I understand the psychoacoustic effect you're going for by emphasizing mids in the center channel. I can see where this might work well for home theater, but I imagine it would sound weird listening to music on such a system. Although I supposed that would just be personal opinion, per listener.Taken at the output of each of the two sections of logic addition followed by a low-pass filter, the level of the summation of the middle signal in the two channels is +6 dB relative to the level of the summation of the left and right signals. Voltage gains and losses of the upmixer are configured so that the level of the middle signal at the output of the upmixer is +3 dB with respect to input level. Thus the level of the summation of the left and right signals becomes -3 dB. From this it follows that the level of the left signal in the middle channel is -6 dB with respect to the level of the left signal in the left channel. The same applies to the right signal.
Cross-feeding of the processor attenuates the middle signal in the left and right channels each by -3 dB. Thus the level of the middle signal in the generated middle channel is +6 dB relative to its level in the left or right channel. This is the same level difference that results when deriving a middle channel by addition of the left and right channels.
The above occurs over the frequency range of 400 Hz to 1600 Hz (two octaves). So the main advantage of the upmixer versus the method of simply adding the left and right channels is the attenuation of the left and right signals in the middle channel. Adding left and right channels results in no attenuation of the left and right signals in the derived middle channel.
Granted that even -6 dB is not a lot of attenuation but it seems to work.
Anyway, you keep saying that you're getting greater attenuation of left and right channels in the center channel, and I don't think it's mathematically possible to achieve what you're describing. I'm almost positive you're fooling yourself on this one. When starting with a regular stereo recording, there simply isn't enough information in the recording to really separate things.
Maybe, if you assume that what you consider the "real" left and right channel information is mostly high and low frequencies, and that what you consider the "real" center channel information is mostly mid frequencies, then your claim would make a little more sense, but I don't think those are valid assumptions.
Don't get me wrong - if this system achieves a desired effect and you're happy with the results, then that's great! I'm happy for you. But I think it involves a lot of assumptions and compromises, and I think the way you're describing the results is probably a little misleading.