After simply accepting this situation for quite a while, I finally realised that the sound levels were too simply too high on some albums. Or possibly the Apple hardware was emitting sound levels that were too high. Either way, it is easily solved. Sound level is, of course, not volume level. It means that the peaks of the sound waves are higher than your speakers can handle.
To solve this problem, use the equaliser in iTunes. Choose the Flat setting and drop the Preamp to -6 DB. Voila; the 12" Powerbook sounds amazing on any albumn (and really folks, this is the best sounding Apple ever).

