In general I really dislike loud music,I don't play rock and never want to play at rock volume levels.from the late 70s to the earlyu 90s I played only acoustic music,and have only returned to electronics in the last 5 years. But in a guitar player magazine article one of the sonics said they had specifically gotten this high volume technique/effect idea from Glen Branca,and that you could only do it at very high volume levels. I like your phrase spectral contours alot. and suspect that you're right that some people have to learn to hear such things.But I do think there' aee things that could only be gotten with extreme volume.I may never know for sure because I'mnever going to play at those levels. In Gamelan music there are related effects ,much subtler ,but Gamelan player end up with hearing damage too ,if they don't use ear protection. To really hear the shimmeryouhave to hit those keys. I've only seen them do an acoustic set (?!?) but I still know what you mean. So I guess it isn't just a high volume effect. I suspect that playing really really loud makes it easier to forget some of the usual things we hear (notes and rhythms), and tune into spectral contours and timbres instead. But once you've done that, you can listen that way at lower levels too. Sort of like how jumping off a cliff might make you forget your stiff neck for a moment, but then after that you can feel pain all over.