As suggested paired Aux1&2 in UC Surface talking to a 32r rackmixer, then used a Y cable (2 TS into 1 TRS) into my headphone amp. Left and right channel are quite different volumes.
UC Surface running on a Windows10 Microsoft Surface Pro for mixing on the rackmixer.
Headphone amp is a 6channel Rolls unit so I tried other channels. same problem. Swapped cables. Same problem. Pan control on headphone amp is set dead center. Also tried Aux out into wireless IEM - same problem. Tried other pairs of Aux (3&4, 5&6) etc. Same problem. Source material is not panned at all - should be centered. Not sure what else to check.
I cannot find a pan control within the stereo aux pair - assume it just uses pan control from individual faders which are all set to center.