Update on this issue : I contacted tech support, they told me of a procedure to "reset" the headset settings by making it run out of battery than charge it up again. At first, nothing change but after an hour, balance restored completely. It lasted for one or two days, then the problem reappeared and I sent it to RMA (I bought it second-hand but they checked the manufacturing date to see if it was still under warranty). In my case, it definitely couldn't have been within product tolerance, I had to put the left cup almost at double the volume of the right to restore the balance.
That being said, client support from Audeze is amazing : fast answers, super helpful and really good contact.