The above Fletcher-Munson Curves show that, unlike speakers, our ears do not have a flat amplitude response, in fact they are not very sensitive at low frequencies and don't become particularly so until 200 Hz or just below Middle C and from then on it increases until about 5 kHz and then decreases again. The best explanation for this is that we're designed to understand speech and do so by filtering off the high frequencies to discern clarity and intelligibility. Voice has a range of approximately 80 Hz - 3 kHz, but we listen mostly from 250 Hz-3 k Hz. We hear what people are saying by using our ears to locate the sound source, focussing them on it and then singling out the sizzly bits to understand what's being said.
In my opinion this shows clearly that we expect clarity and intelligibility from whatever we are listening to and amplitude is just one of several factors that enable us to achieve this. However as we age, so our sensitivity to higher frequencies deteriorates and by thirty five an average male's hearing has lost approximately 10 db (reduced by a factor of ten!) at 3 kHz. Although he may not realise it, he will be finding it more difficult to hear what people are saying or to discern clarity from his hi fi.
Our ear canal produces distortion, tests indicate this to slowly rise to about 8% at 105 dB and we use this to tell us how loud something is, but as we age and our hearing deteriorates and it too is distorting, so we tend to perceive sound as louder than it is and find it more difficult to tolerate. We need clarity at low levels if sound/music is to be relaxing to listen to.
We also have a sort of automatic volume control that can turn down the ear's sensitivity if sound is too loud for us, which is why young people are warned not to listen to iPods at high levels. The process is called temporary threshold shift and it can become permanent if we are exposed to high SPLs for prolonged periods. It probably accounts in part for thin, bass light recordings of some older rock music of which Led Zeppelin might be an example. The producer may have been playing much too loud and so added treble or removed bass to compensate for threshold shift.
Anyone can test their hearing on the net these days and it might be a useful exercise to compare yours with a notional average and to get an idea of how well or poorly you hear. Women tend to be about 6 dB more sensitive than men and a 10dB drop is colossal, so it's not surprising that young people are more likely to tolerate distorted sound than their elders or want more bass to lessen much stronger (to them) treble.
The ears are merely a means of gathering information, which they pass to the brain for processing, so the better the information they are able to gather, the less the work the brain has to do for it to be intelligible. It's hard work listening to indistinct sound.
Sine Wave Speech tests show how the brain is able to process quite distorted sound to get what it needs from it. If you do the test, play the sine wave speech first, then the proper recording and finally sine wave speech again. The first time you hear it, it will just be an incomprehensible noise, but once you know what has been said, it becomes intelligible. I'd guess this is the process of "running in" hi fi that the subjective brigade sells.
Sine Wave Speech probably explains more than most realise. For instance it could explain why most people are convinced they have the best hi fi and that nothing else is as good, or that reviewers who describe a system in very negative terms on first acquaintance, but having improved after a period of "running in", or why it is that often when people are offered significant improvements, they don't always recognise them. They have subconsciously "programmed" themselves to hear music through their system and need reprogramming before they see benefits of another. Good headphones and an ipod are a tremendous help in providing an accurate reference, which is why professionals use them to listen out for detail speakers miss.
The ears, first and foremost need clarity, but the brain can process a poor information from them to achieve the same result. Clarity becomes more of an issue as we age because of lost HF sensitivity, which means we have to concentrate harder to hear whatever we are listening to.
Therefore hi fi, which is probably part of the process of relaxation, needs to be clear and probably at lower levels than in real life for maximum enjoyment.
It follows that adding treble by tone controls or through distortion in the replay system may also add clarity, but often it just makes things harsher and less bearable, which may explain why some hi fi enthusiasts think they don't like clarity.