I remember reading an article during the early days of surround (DLP), that changing the tone controls DO affect the surround processing. Since DLP's four channels are derived from 2-channel input, I can see where this could be true. However with most current formats where each channel is discrete, I don't see, in general, where the tone control would effect surround processing. If the speakers in the system are identical, I don't see where the control is needed, but if different, can see where it could be used to match the other speakers.