Hello and thank you for your suggestions, I never would have thought of your first one! : )
Sadly it didn't do anything.
Indeed I thought I could simply use a balance to even the two channels, but I do use the direct headphones output to practice and still, there is that 2 db difference.
I contacted Crumar and had some advices from Guido, but the conclusion was that the two outputs should be at the same volume and that I could ask for an exchange to my seller. I think that's what I'm gonna do.
I won't lie, I'm disappointed to have a problem straight out of the box. But to be fair, I had an issue when I bought the Prophet 6, had to exchange it, and it remained one of my favorite instrument ever since so... I can't let that kind of issue ruin the great sounds of the Mojo (trying to keep a positive attitude, hum).