On older versions of X-AIR (XR12/XR16) Firmware there were some phase issues when stereo linking channel 9 and 10 for example, one of the channels phase would be wrongly flipped and no way to change it individually when linked so the audio would loose it's stereo coherence and center audio sounded panned. This has now been addressed since Firmware version 1.10, as you can see below in the release notes. If you are having this problem all you need to do is upgrade the firmware to V1.10 or above.
XR16-XR12_ReleaseNotes_Firmware Version 1.10
• linking of line input gains fixed
• correct indication of 0 dB fader position
• XR12 ch 1-4, 10-12 phase fix
• XR16 ch 1-8, 14-16 phase fix
Downloads are available on the respective product pages: