Hi BobG,
In terms of "performance difference", it kinda depends how you define that. If your amplifier produces 225 watts per channel into a 4-ohm load like the M80s, then the amplifier will indeed be delivering more current in amperes (because the 4-ohm load offers less resistance, so more current flows) and hence, referencing Ohm's Law, more power.
So you could quite legitimately boast to your friends that your amp puts out 225 watts of power (at peaks) into the M80s.
Now, if you had a pair of M60s (8 ohms) hooked up to your amplifier and it is delivering its maximum of 125 watts per channel, and you had a second identical power amplifier driving the 4-ohm M80s at 225 watts per channel, all other things being equal (which they never are), you'd still get roughly the same "sound" or acoustical output (loudness), give or take a few dB. Actually, the M80s would be a little bit louder, about 2 dB, than the M60s because the M80s have slightly greater sensitivity. That means that given 1 watt of input, they produce slightly louder acoustic output than the M60s, by 2 dB.
And of course if you have the M80s, or plan to get them, you could also boast to your budds that they'll handle up to 700 watts per channel of power.
Regards,
