FWIW, on my REV2-16, the right output is slightly louder than the left one, on both A and B outputs (even when Mono is selected in Global menu Stereo/Mono), but for eight of the sixteen voices only.
For example, I select Mono in the Global menu, I then call the Basic Program and play the same note successively, and for eight of the notes my left and right VU Meters on my audio mixer are showing identical levels (Master Volume level on REV2 adjusted for 0dB), and for the next eight ones the right channel is showing about +1 dB. If I continue playing successive notes, it goes back to showing 0 dB for both channels for the next eight notes.
I'm running OS 1.1.0.3 and have done several calibrations with the synth fully warmed-up. No change.
BTW, there is nothing connected in the MIDI jacks at all, so no possibility of external MIDI data interfering with the panning.