Hopefully someone can pursue the idea of physically disconnecting the amps to get the performance for the L/R in all channels. However, it would seem to be important that you need to know what your are doing, and that things like amps can be very dangerous. For the average person there would be no way to know if opening up the receiver and disconnecting the amps would be safe. Hopefully someone with the knowledge of such things can investigate it as it is really interesting if it could be possible to for example to physically disconnect the amps you want to externally amplify and get performance that is significanly better than the current AV7705 and AV8805 as well as other processors. I would certainly urge caution if you don't know what you are doing (such as myself).
From having designed amplifiers (at radio and microwave frequency) I've been thinking of the various reasons for the preamp performance degradation when the power amp is swinging to large amplitudes and came up with a few hypotheses:
1) Large power amp swings are causing problems with the power supply - the evidence is against this as Amir measured good SINAD when other channel amplifiers where putting out high power levels
2) Parasitic coupling from the PA back to the preamp - either capacitive (more likely) or magnetic.
3) A large swing stability issue changing the input impedance to the PA under high swings
4) Some protection circuit activating causing non-linearity in the preamp or the preamp loading
It may be possible to disconnect the amp stage but might not be too simple as there could be some protection circuit that will detect this as a problem and interfere with the functionality of the AVS overall.
OK, with that said, who is going to try?