iMac Pro audio clips when using USB as the output

My 18 core iMac Pro is a lovely machine however I've notice in the last few months that the audio output is clipped when listening to Apple Music via a USB-C port. So the connections are: USB-C on back of iMac Pro goes directly into a (bought new 1 year ago) Burson 3XGT USB input port. The 3XGT is a combo Amp/DAC. When listening to certain songs with moderate bass content the bass is definitely clipped (sounds "ragged" and distorted). It sounds as if the input volume is overloading the amp but that's not the case. When listening to the same songs with the same setup but from one of my MacBook Pros or my new MacBook Air, everything sounds perfect even at much higher volumes.

The USB-C cable is OEM Apple and I've tried several (all original Apple) and the listening volume is set fairly low although I've tried lower and higher with no change). I've tried several different headphones an the iMacPro is always clipped but none of my other Macs have this issue. Any ideas?

I've already power cycled the machine and reset the PRAM

Susan