Amplifier Specification questions

I understand the differences between different classes of amplifiers. Most common amplifiers are Class A/B. Does this mean that all class A/B amplifiers operate in Class A to some point then switch over to Class B when they hit a predefined threshold by the design of the manufacturer. Do some so-called Class A/B amps only operate in Class B? My power amp is designed to operate up to 18 watts Class A before changing class of operation. I assume this number is in 8 ohms. My amp is rated at 150 watts into 8 ohms and 300 into 4 ohms. Does this mean that Class A rating will double to 36 watts in 4 ohms or will it stay at 18 watts regardless. What's funny is I was thinking I needed more power because my meters were always pretty well deflected but after looking closer, I realized the meters were on the 5-WATT scale instead of the 150-WATT scale. I can't believe I never noticed that before.