No Chris, you shouldn't assume a problem which would probably never develop and sell your 575. Lab tests have shown that the recent Onkyo receivers have substantial power capacity at 4ohms. Even if you should decide to go with the M80s, the probability is that it'll be just fine. Any receiver can drive a 4ohm speaker such as the M80; the question is how loud and how long. Again, there's no good reason for you to assume that your use pattern will be too severe for your 575.

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Enjoy the music, not the equipment.