This is one of those situations that will drive many audio-enthusiasts crazy. It really depends on how much work you want to put into this. You could simply keep the DAC off (along with all LED lighting) while listening to your tuners. That would be the easiest solution here.
Or, you could unplug all audio interconnect cables from the DAC (with the unit powered ON), and see if that makes a difference, and report back your findings. It’s very possible you *might* be able to reduce this RFI using ferrite on some of the DAC cables (which ones will need to be determined). One tuner (the Magnum) might be more sensitive to this RFI vs your other tuners.
Hmmm, an audio-enthusiast that still values OTA (over the air) FM listening. I'm impressed.