Randall century 200ii 250 watts into 150watt 4x12=crazy high pitched sound?

my 4x12 is 150 watts with a peak wattage is 240, now i just bought the amp head i didn't see it was 250 watts when i plug it into my cab and turn on the head it nearly blows the speakers making a really annoying loud sound. i was wondering if its mismatching watts or if its my amp head that's messed up. I cant test if its the amp head because i don't have a 4x12 with enough watts to handle the head so i cant tell if its broken or its overpowering the cab and if i get a 300 watt halfstack should i be okay? anyone have any experiences or opinions on the head i have? its a randall century II