Use one DAC. Export the signal from your PC in digital (I'd personally go with the USB type B straight to the speakers, but other members might disagree, if the sound card of your PC has Toslink, you might use that as well, but by all means export it in digital) to any of the three. Once converted just amplify it using, of course, the amp in your active speakers. It is a customized amp made to work perfectly with the speakers.
If you choose to go through one of the external DAC's, you should also go straight to amplifying after the DAC. Don't get me wrong, but I don't like the output connections of those DAC's. Mojo is just 3.5 jack and Young is only balanced (not that I don't like balanced, but you speakers have RCA Analog Line Level ins).
This is what I would do if it was me:
I'd slap these adapters and connect to line level on the KEF's
if I was for some reason using the external DAC which I wouldn't, I'd go PC to KEF.
But my first option would be to use KEF's inbuilt DAC. Each channel is converted separately and I doubt there's anything lacking in those DAC's.