Too true. And every DJ or musician thinks they are a sound engineer by
default.

You could offer the venue owner a job though - phase mis-alignment
isn't the easiest thing in the world to hear (in my experience about
75% of people can't even hear it).

In answer to the original question though - you say the speakers are
"used to seeing" about 100watts - is this per speaker, and if so how
many are there?

Crown have a tendency to power rate their amps at 2ohm impedance loads,
where *most* speakers are 8ohm. this would translate to roughly
400watts, split into 2 8 ohm speakers, so about 200watts per speaker,
which is not a bad power rating for a 100watt rated speaker (most
speakers can handle transients up to twice or 3 times their RMS spec -
the thing that destroys speakers more quickly than anything is
distortion which you are more likely to get with an underpowered amp -
or a mentally challenged DJ who thinks red lights are pretty).

Of course, if there are 2 or more speakers for each channel, you could
also connect them in series instead of paralell, giving a 16ohm load at
the amp output instead of 4, and another drop in the total power.)

BUT - as I said, the thing that will destroy your speakers is
distortion (you can burn out the coils by overpowering too, but
distortion is much more of a problem), and distortion is most likely to
be introduced before the signal ever hits the amplifier. Keeping the
amp turned down can limit this to some extent, but best practice would
be to educate the people using the equipment (mostly DJ's I assume),
and impress on them the benefits of unity gain (0db signal levels) from
their DJ mixer. I find a stick with a few nails in it to be the best
method generally...:-)