If my speakers are rated at 150 watts, does that mean that a 180 watt amplifier is not the correct choice?
Not necessarily. A 180 watt amplifier simply means that the unit can output that much power, not that it will do so all the time. If you have speakers that can handle less power than the amplifier’s maximum, that is fine. Just do not turn the volume control on your preamp/processor all the way up. The extra power capability will come in handy when you upgrade you speakers. It is also better to have an amplifier that has more power handling capability than not enough; this can prevent clipping and distortion when driven to brief, loud levels such as in DVD action/adventure movies (crashes, gunfire, explosions etc).