DAC / USB outputs in MONO? HELP

Hello,

At 73 I'm attempting to upgrade my office system and I can NOT seem to get a stereo signal out of my DAC? I have used various on line samples that are "in phase" and have audio. When I attempt an "out of phase" signal I get nothing from either channel? I have also tried "spoken left-right-center" samples and all these samples are the same on both channels?

System configuration:

Desktop PC Win10 (Amazon Music) with 20ft USB into a SMSLSu8 DAC, direct into Threshold 400A amplifier driving Magnepan LRS speakers.

Any help folks would be greatly appreciated...

Cheers' Darrell