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View Full Discussion A lot of people think amplifier power requirements are lower for smaller speakers. The opposite is often true. Many bookshelf speakers are less sensitive than larger floorstanders, which means the little bookshelf speaker will need more power to play as loudly as the larger speaker. Sensitivity is what you need to look at, not speaker size.
I run a pair of speakers which have a(n admittedly better than average) rated sensitivity of 94dB/watt (at 0 distance from the speaker). I use an amplifier that won't deliver any more than about six clean watts. They'll play eviction-loud.
Crown has a nice amplifier calculator on their website: http://www.crownaudio.com/en-US/tools/calculators
An 86dB sensitive speaker, which is typical of an average not-very-sensitive bookshelf, will still play at 85dB with about 3 watts at a distance of 2m. That's not quiet.