Over the decades, I’ve come to be of the opinion in general that detail is not really at all related to brightness - except in systems plagued with either electrical noise or (as previously noted by others here) certain types of distortions, or both. There are no systems extant that are free of either noise or distortions - but overall I do think detail is more directly related to presence...not just midrange presence, but presence across the board and that extends well into the treble. Detail and brightness are not inherently mutually inclusive or there would be no systems at all that would sound correct to our ears in this regard.
OTOH, I suspect Geoff’s observations above of the current state of systems’ health mirror most everyone else’s observations and that they’re indeed somehow lacking on this point. I know that when you go to increase the EQ in an attempt to mitigate lack of detail, you’re faced, consciously or otherwise, with having to settle for a "best-compromise" setting (no matter what the quality of the EQ) between frequency increase and an increase in that same range of noise (hash). You might prefer having the new EQ setting to not, but the EQ’s usefulness will always limited by the compromise made by noise...that is until you manage to get rid of the noise - noise you might not have been aware was so dominant in the (lack of) performance in your system that you would’ve been prepared to say had sounded rather good, if not better than that. If you can get rid of enough of it, then the EQ can be at Any setting (including maximum) and all that you will hear is an increase in frequency only...no hash, no bleeding ears, no "bad" recordings, no time or phase distortions, no nothing. This is the point at which the system has become truly linear.
This same principle applies to video as well. As you turn up the sharpness control, the image will become increasingly hyped until noise begins to limit the HF resolution and then true image detail begins to erode the closer to maximum you get. After removing enough of the noise, all that remains is true picture detail, no matter what the sharpness control setting...you just now need to decide what overall level balance is most pleasing to the eye...a truly linear circuit.
It’s just that few people have ever witnessed a "truly linear" system. They have been quite hard to come by so far and are quite rare...but it is possible to attain.
(Note that I’m saying that "electrical noise" is a universal phenomena that no system, no matter how well treated is fully exempt from and that "distortions" are system- or gear-specific and that none of what I’ve said here is taking into account the acoustic distortions in the room itself.)
My 2 cents.
