You can safely use a 300 watt amp on 600w speakers.
If you had a 600 watt amp the speaker would only be able to play 3db louder which isn't a big difference. There is an equal difference in volume when you go from 1 watt to 2 watts as when you go from 300 to 600 watts. You have to keep doubling the power to get the same increase in volume.