Incidentally, it can be pointed out that the amplifier doesn't "see" a lower impedance when it's bridged, as is sometimes said. The impedance of a speaker is set by its wiring and the amplifier can't change this.
Last year I emailed Alan to ask about M80's, bi-amping, and impedance, as part of the whole Emotiva vs. Axiom impedance debacle. My question was actually featured in the April 2009 issue of Axiom's Audiofile newsletter.
Ian answered in that newsletter,
"The x-over in a speaker keeps the impedance of a multi-driver system from being averaged down over the various driver impedances. Bi-amping simply separates some of the frequencies from others but does not necessarily change the impedance load. It will have some effect and depending on the speaker can change it from a 4-ohm load to an 8-ohm load but that would be rare. In an M80, it does raise the impedance of each separate channel but not enough to take it out of the 4-ohm category."
So to Bi-amp, one must remove the bridge plates. And removing the bridge plates alters the 'system'. That, according to Ian if I'm reading his response correctly, raises the impedance 'seen' by the amp. In an M80, at least. Therefore, bi-amping can and does change an M80's impedance. Not the act itself, but the setup *for* bi-amping.
Of course, the OP is talking about an M60, so I have no idea whether that applies. I'd be surprised if it didn't, however. The best source would be to email Alan directly, as he'll know.
My M80's are 'pseudo' bi-amped with my Emotiva LPA-1. I've tried it both ways, but prefer it bi-amped. Just sounds better to me. I will not debate that it is possibly, likely, or definitely just a psycho-acoustic effect, all in my head. That's all the enjoyment of music is anyway.
My advice is to try it both ways. If it's a 'free' feature of your AVR, and you've got the channels open anyway, your only expense in trying is the extra speaker wire and the time it takes to hook it up. It'll either sound better, exactly the same, or worse. Listen to your system in your room on your gear
and decide for yourself.
Good luck.
