Thank you for helping me troubleshoot this issue! Here are photos of the solder points and the audio wiring configuration. I did modify the box design but otherwise followed all of the instructions in the DINAS build book with the recommended hardware.
Here is a link to a video I recorded so you can hear the humming noise. The noise is only coming out of the Daytons. The sound happens with the line in of the amp connected to the MacMini with no audio signal playing out of the headphone jack.
I just reluctantly tested a speaker on my laptop and found that there was no hum with one power adapter plugged into one amp.
With both power adapters plugged into the amps and one amp powered on the hum returned and with both amps powered on more than doubled in volume. Same test result on my tablet headphone jack. Testing the MacMini the hum is still audible with only one power adapter connected to one amp.