Distortion NAD C 658

Thank you for reading this post; I am having an issue with distortion while using the NAD C 658. Through the BluOS app, I hear speaker distortion regardless of the streaming source. The distortion is exacerbated by turning the app volume up to 100%. I hear it not matter how loud my McIntosh 6700 is set to. I have replaced the NAD C658, but the issue persists. When I switch sources on the McIntosh 6700 the problem goes away. I hear it on both speakers. Any thoughts?

System info:
Streamer/dac NAD C658 XLR output into McIntosh 6700. Speakers are Revel 228 Be.

Thanks everyone.