Volume uneven from monoblocks with balanced setting, but not unbalanced.

I may have bought a faulty set of preamp and monoblocks, though I want to ask if it's a small or big fault. Sorry for a lengthy description.

I bought an Exposure 5010 pre and a pair of Exposure 5010 monoblocks second hand from a seller in Norway while I myself live in Sweden. I have at home noticed an imbalance in volume between the monoblocks and have ruled out bad cables and power source by swapping from right to left one cable and unit at a time and retesting by listening to music (mono recordings). Swapping the monoblocks while keeping everything else the same shifts the imbalance. I have the monoblocks connected to the pre through balanced xlr cables. I have made sure both selectors for balanced/unbalanced are correct on the monoblocks. The problem is independent of the volume pots position on the pre.

After some further troubleshooting, I found that opting for unbalanced rca cables and selecting the unbalanced setting on the monoblocks fixed the volume mismatch. Going back to balanced cables but keeping the monoblocks in the unbalanced setting also resulted in an even volume balance. So the issue seems to be with the balanced setting on one of the monoblocks. Also using unbalanced cables and flipping the switch to balanced creates a volume mismatch, though I can't remember if it was the same side as when using XLR cables.

Does the presumably faulty setting merely add/subtract 6 dB of volume, or is it more complicated than that? Should I just go for unbalanced interconnects and settings, or is this a symtom that indicates that the sound will be degraded even if the L/R-volume is matched by going this way? Can I do balanced signal through XLR but leave the button in the unbalanced setting, since I already bought the cables? Are there any advantages or perhaps disadvantages/risk of damage in doing so?

Thanks for reading.