itm wrote he bought an XLR/TRS cable, so – since the RME's input is TRS balanced – it seems he now uses the KPA's balanced output?
Yes. As far as I have understood the whole matter, the XLR-outs of the KPA are balanced, but the TRS-outs are not balanced. I don't know what kind of cables he used before but I guess he used stereo TRS cables. You can't use those to connect unbalanced to balanced TRS without loosing half of the signal.
It should work with a mono cable or with a stereo cable plugged in only halfway in the jack.