MacBook Pro USB port audio out issue

Greetings,

Please see below for system specs.

When using one of my two USB ports (doesn't matter which) as an audio output to connect to a DAC (digital to analog converter), anytime I plug a monitor in, it kicks the USB port I am using for the audio-out off. It will intermittently come back on and off for a second or two but always kicks it back off. Once monitor is removed, the audio out via USB works fine.

I've also noticed that if I even touch the metal end of monitor cable ( thunderbolt or HDMI ) to the metal MBP frame, the result is the same. It kicks the USB port offline.

Can someone please shed some light on whats going on here? Specs and what I've tried are below.

**new discovery** - I plugged the USB cable for the audio out into the apple external keyboard and that works.. port is active and I have monitors... that is strange, any thoughts?

Thanks in advance!

Chris C

I have tried:

different/new USB cables to the DAC

different/new DVI with new thunderbolt dongle

different/new DVI to HDMI cable

running the system with the MBP charging cable unplugged and plugged

I've isolated each component ( stereo / MBP / monitors ) to dedicated electrical circuits within my home

different/new power conditioner

different/new power strip surge protector

no power conditioner

no surge protector

different monitor
every different port available on the computer.

System Specs:

MacBook Pro: (Retina, 15-inch, Mid 2015)

Processor: 2.5 GHz Intel Core i7

Memory: 16 GB 1600 MHz DDR3

Graphics: Intel Iris Pro 1536 MB

OS: High Sierra v. 10.13.6 (17G65)