P, Ken's reply should be "less nuanced" enough, so I'll just point out that impedance ratings and suggested maximum amplifier power don't "interact"; they have nothing to do with each other. The maximum amp number is essentially meaningless for home use; quality speakers such as Axioms can continuously handle many hundreds of watts without suffering permanent damage, but your hearing can't.
Incidentally, I did take a look at that ST-500 site link. Unless they operate in a world where Ohm's Law doesn't apply, the 550 watt 4 ohm spec is a typo. As was said, the theoretical maximum increase is a double, so possibly the spec was intended to be 500 watts.
