Well aside from the speaker's efficiency, other factors include how loud you like to listen, how large and how well damped your listening room is, how far back you sit, and even what type of music you listen to.
For instance, you'll clip on solo piano at a much lower average SPL than on rock music. Piano is extremely demanding of peak amplifier power.
Paradigm's recommendation of 15 watts minimum looks reasonable to me. That would theoretically give you peaks in the lower to mid 90's at 10 feet away in a medium-reverberant room. If possible go with a tube amp, as they can be driven into clipping much farther than a solid state amp can before the distortion becomes audible because they "soft clip", producing fewer high-order harmonics (which is what the ear objects to). In practice, a tube amp will play maybe 3 dB louder than a solid state amp of the same power before you notice the distortion.
Duke
