Strangely the issue seems to be almost gone since I swapped my headphones earpads from L to R, but there is still a minor difference (still in the Left channel).Does it happen on both outputs?
I tried with my own custom earbuds I made and I didn't see any difference from L and R outputs this time.
So I guess is then a minor channel sound output difference from my brand new ATH-MSR7b headphones then.
Everything is good from my unit then
Thanks for your inputs guys

*edit*: just figured out it was a pad seal issue from my headphones, the R side seems to have a bit less seal than the L one providing this imbalance issue. MSR7b is very sensitive to that, some people seems to have this issue aswell.
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