so just i get a 5150 (120 watts) head and put it on to a cab with 150 watts and crank the volume will it blow? is it better to get a 300watt?
It'll be fine.
No, it'll be ok. I warn you that your hearin might not be. But that's your hearin, so do wat ya want with it.
You'll be fine. Just make sure to match the ohms first.