So assuming a medium-sized room (18 x 34 feet) and lower listening levels (never above 90 dB) how much power does one need for the M100s? Or to phrase it differently, how little power can one get by with?

With an (honest?) efficiency rating of 88 dB (1 w / 1 m), and a listening position approximately 3 meters away, the 1 watt SPL should be ~81 dB. (-6 dB for every doubling of distance - so 88 at one meter, 82 at two meters, and 76 at four meters).

So doubling the power for every three dB increase, to get 90 dB at the listening position, I'd need 8 watts of amplifier power (81 = 1W / 84 = 2W / 87 = 4W / 90 = 8W).

Now for SPLs > 90dB, the power requirements increase by a LOT -
93 dB = 16W
96 dB = 32W
99 dB = 64W
102 dB = 128W
105 dB = 256W
etc.

So my conclusion here is that any 50W amplifier (into four ohms) should be more than sufficient for my needs.

Is this correct?

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