Artemis

If you still want to use those HD600 and are into DIY, you can buy a matched set of drivers from sennheiser and replace those old drivers. Also if you feel comfortable and have the time, do open up the headphones and check if the drivers don’t have any foreign materials on them, like hairs etc, as that kind of stuff can impact driver performance. Yes, cleaning IEMs is certainly a thing, they do get clogged as time goes by. Have my best regards and enjoy your music. 🎩

Being young doesn’t really matter as you can also damage your hearing if exposed to loud sounds, but I digress. Regarding channel imbalance. If your amplifier uses old school potentiometers channel balance is certainly a thing. So you indeed need to test that imbalance with other sources. Also I have been enjoying music since my teens and I’m now in my forties, imbalance with headphones was a pretty rare thing for me, as for IEMs besides the rare QC thing I never had that kind of problems. I know it’s a long shot but you should test your amplifier section for DC offset…most amps filter it but one is never too cautious as it can ruin gear.