Speaker Sensitivity Question...

In theory, you're correct. In practice, though, its very difficult to do things that way. Power ratings on amps are not consistent at all. Every manufacturer has their own standards for how they measure power. For example, one company could rate an amp at 100 watts/8ohms. A different company that has higher standards may rate the exact same amp at 75 watts. And another may rate the same amp at 150 watts. If you are new and are having trouble selecting an amp, you'll need to gain some experience through experimentation. Also, talk to the people that make your speakers. They should be able to recommend something that will work for you.

For most music most of the time, a considerable majority of the power that is required will be required at frequencies greater than the crossover frequency that is likely to be used with a powered sub, e.g., 40 or 50 or 60 Hz or thereabouts. (I'm assuming you are referring to a powered sub). As a rough approximation, I believe that 300 Hz is a frequency at which equal amounts of power will often be required at higher and lower frequencies. So in typical circumstances I wouldn't count on a significant reduction of the amplifier power requirement as a result of using a powered sub, crossed over or not.

Also, if the crossover function is being performed "after" the power amplifier, as would be the case if it is performed by the sub itself (with the power amp being connected to the sub, and a high pass filtered output of the sub being connected to the main speakers), the power amp will still have to output a voltage range corresponding to the full frequency range of the signal. So if you were to go to a lower powered amp based on the expectation that it would not have to supply as much power as the full range signal would otherwise require, you still might find yourself clipping the amp if its voltage swing capability is less than what a higher powered amp might be capable of.

ZD makes good points as well. Regards,
-- Al

Zd542 is right about power ratings of amplifiers, they're not always a reliable indicator of true performance.Some amps rated at 50 watts per channel will clearly out drive some amps that claim 100-150 watts per channel.There are numerous factors that determine actual speaker load drive ability.Power supply "quality"(and output transformer quality) isn't always reflected in the measurements. Yet these two variables are very important.
Charles,