The way I understand it you can run a pair of 150W (30 nom) speakers off
an
1500W (600RMS) at full volume without damaging the speakers.

Is this true?

Not generally. If the amp is truly 600 watts and the speakers can truly
handle only 30 watts, the chances are quite good that you'll blow the
speakers.

However, the amp you got ("Starsound") is probably actually rated about
50wrms x 4 at 4 ohms, huh? So much for a 1500 watt amp.