Joe wrote
If high sound levels aren't required, that might not be true.
Assuming that each setup won't be pushed to distortion, decent
speakers driven by 15 watts per channel RMS will probably sound better
than so-so speakers driven by 40 watts per channel RMS.

Regards,
Joe

It's the midbass that suffers ... the head unit just won't kick up enough
brass to maintain a solid level of sound across the frequency spectrum.
Remember that you will have the head unit up near max volume levels to
acheive 15W. Distortion will be just mad.

--
Regards,
Dan Snooks