XM6 does not support audio over USB-C (The USB-C port on the XM6 is only for charging) QC Ultra 2 does, but the headphones turn off when plugged in and have to be turned back on while charging

AirPods Max with USB-C and AirPods Max 2 appear to be the only headphones among this group (XM6, QCUltra2, AirPods Max with USB-C, AirPods Max 2) that support both Audio over USB-C and automatic charging while in use.