It sounds like the MFJ PS is a switch mode type and is not filtered as it should be. External filtering may work but you’ll have to buy several large ferrites to do it. If you know the voltage and current the MFJ requires, you could find an analogue supply instead.
I did more troubleshooting and found even with the 7300 off when I attached the positive lead I lose my audio. I can turn the power supply on with no problems until I attach the positive lead. The only thing I can think of its to try grounding the pc. It would appear the power supply isn’t the problem. Sure has me scratching my head. This problem was apparently there before I installed the software and just had not seen it.