Can you use a 600 watt amp on a 300 watt speaker?

AS long as the impedance is a match you can use any speake with any amp. But if the power ratings are too mismatched don't expect it to sound that good. If the amp is stronger than the speaker, the speaker can be damaged if you crank the amp up high. And if you hook an amp to a too strong speaker the amp might have trouble powering it properly.