I have a 4 channel amp. 2 of the 4 channels will be bridged to create a1 x
300W rms. I want to use this to drive a sub.

I want to experiment with it first. The way I understand it I should buy a
sub with a higher power rating than 300W and then it would be basically
impossible to blow it with the amp? Am I right?

What power rating should I use for the sub considering the fact that a
square wave delivers more power.

Would it be possible to blow a 400W rms earthquake sub with this amp?