There is some ongoing discussion about the minimum phase nature of loudspeakers, but the largest consensus seems to be that, on the whole, they behave as minimum phase systems. Group delay is usually defined as the delay of a signal envelope. One excellent paper on the subject was written by Dr. Leach of Ga. Tech. I personally think Leach has the best explanation of the problem I have ever read. In order to minimize differential delay we need to avoid any type of delay that would allow the waveform to get out of alignment with the envelope of the waveform packet.
Leach shows some representative differential delay responses for various high-pass filters.
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