[ksk]

>(That is, I would expect audiophile-grade equipment to best mimic the monitors on which most recordings are mixed, which presumably is a flat loudness curve.)

Not necessarily, because that assumes the audio was mixed correctly to begin with. The audio will have to be played back over digital and analog broadcasts with varying dynamic range characteristics, not to mention various output technologies, including $5 headphones. And not to mention how the shape of each persons ear affects the response curve in different ways.

There is no way for an engineer to produce a rendition that sounds great in all of those scenarios, making 'mimicking the monitor' a pointless pursuit to begin with - even IF you were an audiophile listening to 400MB DSD files.

>Does anyone know why Apple finds it needed to further apply sound shaping to recorded sound?

I'd say the vast vast majority of people want more to have the audiophile label/association than actually listen to music with a flat curve, because most music will sound like crap.

Sorry for the rant :P