My advice, take it or leave it: is to note what the speaker manufacturer recommends as a minimum power recommendation into the 8 Ohm load of the speaker, then note what the minimum (not nominal) impedance is, then double down the recommended power for each halving of impedance e.g. 10 Watts into 8 Ohms, 20 Watts into 4 Ohms, 40 Watts into 2 Ohms, etc.. When faced with a non-standard impedance, drop down to the lower standard amplifier rating, for example for 3 Ohm minimum speaker load go to the 2 Ohm standard amplifier ratings unless the amplifier manufacturer offers non-standard ratings (such as the Benchmark amps). Using that as a minimum power need. While room size and desired output levels will actually determine how much power one will need, as a general rule of thumb for most people, in most rooms will most probably be better suited by doubling the manufactures minimum power recommendations into that minimum impedance as noted above for satisfactory results. The important thing is actual power into that lowest impedance. I don't recommend relying on so called "stable into X load " claims, as that only means the amp won't go into osciallation when presented with that impedance. it is not a guarantee of power output or distortions levels into that load. Don't be swayed by manufacturers, dealers, etc., annecdotal suggestions; let them put it writting. Using the above considerations can go a long way towards preventing speaker damage, and offering satsifying sound.
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