I don't see any A-150 on their website. Strange.
The A-300 is 150 w/ch, so unless you were really topping out the speakers and not running a subwoofer it's going to have plenty of headroom. Like a motor in a car, I don't know that anyone has ever regretted having a little extra power on hand. It'll run cooler than a taxed amp of half the power (if that's what an A-150 is.)
BTW, if you would ever upgrade to bigger/better speakers the amp will handle it easily.