redget,
chesseroo would give you a more definitive answer, but I suspect that the amp's impedance setting only changes the stringency of its current protection circuit. You can certainly experiment with both settings using your existing speakers, to see if it changes the sound quality at all.
That said, and given that your Onkyo officially supports 4-ohm loads, I would not consider the impedance rating as a primary factor in choosing between speaker models. Moreover, I understand that the M80s are more efficient than the M60s, which will reduce the amp's actual workload.
Cheers!
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Uh-Oh, here is an Onkyo owner who has been able to engage the protection circuit with the M80s... Well, I do not know what to read into anymore... After all, a total rated power of over 700W was simply unthinkable from relatively compact all-in-ones 15 years ago. It may be that we are just expecting too much on those poor receivers.
