Originally Posted by
davef
The impedance rating of a loudspeaker does not affect the power ratings for the loudspeaker itself. In other words, if the speaker is rated at 100 watts max input power, it would be 100 watts max regardless of the impedance rating of the loudspeaker.
Your amplifier does not need to be rated into a 4 ohm load as the Sierra Towers are a rather easy load to drive. In addition, you should not purchase an amplifier based on the power handling rating of the speakers, even a 50 watt amplifier will drive the towers to extremely loud listening levels. The power handling rating of a loudspeaker is simply a guide to what is the maximum recommended power you can feed the speakers. Realistically, most consumers will never feed more then 20 watts continuous into the towers as even this seemingly small amount of power will produce over 96dB (extremely loud) at 8 feet back in an average listening environment. There is no difference between 20 watts from a 100 watt amp or 20 watts from a 300 watt amp.
Choose your amplifier's power output by the speaker's sensitivity rating, how far back you will be positioned from the speaker and your listening preferences. Considering dynamic headroom capability, 105dB max peak output is about as loud as anyone should ever listen at. With that in mind, assuming you are listening at 8 feet back, 2 speakers, and accounting for room gain, 100 watt amplifier (rated into 8 ohms) is a very good choice.
Hope this helps!