Charles, if by "separation" you mean the degree to which signals in one channel of a 2-channel amplifier or receiver are attenuated with respect to the other channel, then this hasn't been a problem for many years and there's no need to consider what has an "impact" on it. Generally, 40dB of inter-channel separation is sufficient for full stereo effect with no audible leakage. Some of the test labs(e.g., HomeTheater Magazine)still include the crosstalk measurement in their tests. I don't recall any unit in recent years that had less than about 70dB of separation and typically a receiver would measure in the 80s(the very inexpensive receiver that I have on at the moment measured about 90dB). Some units have 100dB or a bit more of separation and this is an indication of excellent design but isn't of any added significance to the listener.
