It's a really difficult subject to be precise about, because the variation among people is simply enormous. Here's my take:
1) Based on long-term observation of Stereophile magazine reviews, I have said for some time that "most audiophiles, most of the time, will be satisfied with a minimum peak power capability of 102dB per channel." This is essentially identical to the Dolby THX specification for home theater systems. SEX has 2 watts, which is 3dB above 1 watt, so a 99dB efficiency would meet this spec.
2) A study in the literature of studio recording and mastering engineers showd that they listen at a maximum level of 82dB per speaker. Other studies indicate that well-recorded analog (vinyl) music has a headroom of 14dB, so an instantaneous peak of 96dB should be sufficient - implying 93dB for use with the SEX.
The difference of 6dB relative to 1) above is easily attributed to explosions and car crashes, common in movies but not in music.
A different approach is offered by some engineers - solid state is said to have a loss of quality in the 6dB below peak power, so the "quality sound" range is 6dB less than the rated power. There are some technical rationalizations for this view, but the important part to me is that it's based on observation - tube amps generally can compete with solid state amps of 6dB greater power. (6dB is a factor of 4, so that's saying an 8-watt solid state amp can be competitive with a 2 watts tube amp.)
3) People's taste in music and loudness is highly variable. That's hidden in the phrase "most audiophiles, most of the time" used in 1) above.
For a small proportion, it is important to approach the sound of a live rock concert - that will take some 12dB more loudness, maybe more. If you do not already have significant hearing loss, a habit of this will provide it. :^)
I think that, to hear the utmost resolution of a system (i.e. not just to hear the music per se) you do need greater peak power capability - that 102dB capability at a minimum. Several audio pros (i.e. more than just music lovers) of my acquaintance really need this capability even though they rarely use it. These are guys for whose ears I have the greatest respect.
On the other side, for the average music lover (as opposed to audiophile) something like 6dB less peak loudness is perfectly fine - you can hear all the musical details, without offending housemates or neighbors. That gets you down to 87dB speakers with the SEX. For what it's worth, I myself use 89dB speakers with my SEX amp, DVD player and TV to watch movies and I've never even approached clipping levels. But then, we have almost every Jane Austen movie ever made (my wife's a fan) and no Bruce Willis movies at all.
Music makes a difference. I love symphonic music, and my biggest stress on a system comes when I listen to Mahler. My main system is 101dB efficiency with 8-watt Paramounts. I don't get to let it loose very often! If you like Brubeck or Joan Baez, you don't need anything like that.
4) Small live rooms need less power than large, dead rooms. Good for +/- 3dB I'd say, unless your room is really exceptional.
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Don't know if that's much help, but I tried to set the stage at least.
