Frosti, of course the answer depends on what you'd consider "good sound". One watt is enough to drive the M60s to a comfortably loud listening level in the low 80s of decibels at the typical listening distance. If you want to be able to reproduce brief peaks on highly dynamic material, such as many classical recordings, you would want to go 20dB higher than that one watt level, which would be 100 watts.

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Enjoy the music, not the equipment.