The thinking behind the series diode is not crosstalk. It's to prevent one amp from draining away the charge on it's sister amps cards. On movie surround tracks there could be an explosion isolated to just one surround speaker. The other surround channels may have other sounds at the same time. Without the diodes, the "explosion" channel could drain current form all the caps then all channels will clip. With the diode, it can only draw off the primary supply caps. If it runs out of current, well then that channel clips but the remaining channels at least heve their 5600uf to draw from which may be fine if the power demands are low at that instant.
I agree the best solution is a power transformer per channel which I wish I did as the giant 600va toriod is going to be a tough fit in the box!