Fred, the recommended max of 250 watts for the M60 would imply that it wouldn't go up in smoke at that input(and probably quite a bit more), but doesn't supply much helpful information as to how much power is needed for a particular use. More useful in that respect would be the sensitivity rating of 89dB. This means that at one meter anechoically(not counting room reinforcement)1 watt would be needed for a rather loud 89dB listening level. Several factors such as room size, liveness, listening distance, etc., affect what the level at the listener's ears would be, but as a rough rule of thumb at a typical distance of about 10', 3-4dB can be subtracted from the sensitivity rating to indicate the total loudness for 1 watt resulting from the combination of direct sound plus room reflections. So, with the M60s 1 watt would be used for about 85dB at the listening position.
Now, the dynamic range of the recorded material(not the speaker)is what's significant in determining how much power is needed to handle brief peaks. Some very loud pop items are mastered with a dynamic range approaching zero, while some other recordings, especially in the classical realm, have a dynamic range of as much as 60-70dB. It's often suggested that to allow for material with a wide dynamic range that 20dB above a comfortably loud(but safe for hearing)average level be provided for. For example, if 85dB(actually louder than a typical average level)was used for listening to dynamic material on the M60s, to accommodate 105dB peaks about 100 watts would be used. So, it depends on how loud of an average level the listener adopts and how much above the average that the peaks hit on the recordings in use.
