I've had a fair amount of headphone experience but channel imbalance is very rare for me. The few times it has happened have been with IEMs that needed cleaning, or once where it happened suddenly and seemed like an actual hardware failure. Are you sure it's not a source issue? Are you sure it's not your hearing?
BudgetAudiophile
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I habe already flipped the headphones so it's nit my hearing (I'm also pretty young so yeah). I will try another DAC at home today, so I'll update this comment once I do that.
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.
I tried my HD 600 at home with different dacs (two ones from Fiio) and they both game me that same channel imbalance which shifts higher tones to the right. So unfortunately this seems to be a headphone thing. I meam/, they served me very well for the last four or five years, I used the loads. I guess its time to get some new ones.. And also I'll look I to cleaning my IEMs.
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. 🎩