P, it appears that you're paying too much attention to nominal speaker impedance ratings and also the maximum suggested amplifier power. Speakers vary widely in impedance as frequency varies; for example, the M80 is a 4 ohm speaker over a bit less than half its frequency range. So yes, at those frequencies where it happens to be around 4 ohms, amplifiers face lower resistance and have a higher maximum output capability. The 250w/550w example you give isn't very realistic though, since the theoretical maximum difference is double, and more typical would be about 50% higher into 4 ohms.
Also, the maximum power rating given to a speaker has nothing to do with how much it actually needs, but rather indicates how much it can handle continuously without going up in smoke. Speakers such as the M80s use about 1 watt at a comfortably loud average sound level. Brief split-second peaks in material with a wide dynamic range, such as some classical material, take much more of course, but rarely would need more than the typical 100 watt receiver ratings.
"Just enough" power might be only a few watts for some material, and certainly wouldn't be the maximum advertised suggested amplifier power.
