Different genres of music can have different typical mixing conventions in the studio- typically the "sound" of a certain genre/style/era may have instruments, sound effects, backing vocals etc that are specifically panned/biased to one channel for instance. So if a particular element in the track producing harmonic content asymmetrically in the mix, and that happens to coincide with some defect in OP's hearing on one side, it's very possible that simply swapping channels could inadvertently "bypass" this irritation since now that harmonic content causing the problem with the bad ear is now reaching the "good" ear instead by virtue of the swap.
As an example, I personally have a hearing notch from 500-1000hz in my right ear which happens to coincide with the first harmonics of a cowbell, which is quite typically panned right along with similar percussive elements in most styles that choose to incorporate it. I can barely hear it over other sounds at all, and if I turn the music loud enough it it extremely distorted and jarring in that region, however if I swap channels in my music player I can hear it perfectly fine in my other ear. Funnily enough I also have a notch in the "good" ear at a completely different band, so a particular instrument playing in certain octaves might sound better in one ear and vice versa and it is largely dependent on the composition of the tune.
From experience this is not really an elegant solution, because if whatever is bothering him happens to come up in the other channel then the problem is no longer solved, and other harmonic content is still being affected by the deficit. It would be a concession or tradeoff he would be making between tracks, bands, genres etc. even if one side was consistently better. So it makes more sense to me to simply identify the issue with a tone gen (and could use a previous audiogram as a guide) and then handling it appropriately with EQ + trial and error, which could involve either completely notching out the problem bandwidth with an appropriately steep filter if it's narrow enough not to affect large swathes of the sound, or with varying degrees of negative (or even positive) amplification in the problem band or adjacent bands.
What many people identify as "sensitivities" with hearing are rarely that in the strictest sense of the word- it is usually either some sort of hearing deficit or damage in a particular range that causes a loss of perceived dynamic range and added noise/distortion, i.e. physiological damage through age (chronic exposure), acute exposure, medications, autoimmune conditions, vascular issues and so on.
Thank you VERY much Jae for this brilliant and highly-informed post, and very helpful reply, -- I am saving this one, and the others to the HiFi section of my bookmarks / favourites ! -- ( "favourites" is underlined by my American spell-check !!)
Up until about 5 years ago, I would not have understood much of it, but I got lucky with a new budget CD player + Headphone Amp + Headphones , that revealed a lot of sound detail for the first time, -- the position of instruments, new harmonic sounds, textures and montages, the decay of a sound into a silent space, intimate hearing of an instrument played very quietly, ETC -- but OMG (Oh My God !) - What's gone wrong with the lead vocalist who appears to have moved towards the back of the stage and has turned sideways !!
I replaced the basic interconnects with recommended ones which luckily gave an overall sound picture or soundscape in-which, to my ears, I can hear all that extra detail without ( the illusion of ) the lead singer being adversely affected -- the overall balance of the sounds was right for MY ears, at least.
-- IF I were to be able to change / reduce one small detail or sound, I might harm the rest.
In recent times I damaged my 5 year old headphones and replaced them with an alternative version from the same maker -- again, a lot of new sound detail was "prominent" / close to my ears, but again this had the effect of pushing-back ( - submerging, etc) the lead singer !
So again, - based on the previous success, I have bought highly recommended interconnects, and again the balance of sounds seems corrected.
Going back to my treble problem, your expertise above Jae may help me on my road to a musical Shangri-La !
I have recently discovered the diyaudioheaven.wordpress.com website, and that could prove to be helpful too.
Many Thanks from KB.