I am experiencing this exact same thing. I just realized it 1 weeks ago. I had both speakers right next to each other and I realized both speakers (same model setting and size were playing my music at different sound levels. A very noticeable difference. I changed cables and even switched them. My left channel comes in lower than my right side.
I am determined to having this fixed. Or if this is a universal problem with all ddj-sz or possible this may have been resolved with an update or does a part needs to be serviced? I would like to know.