I disagree that you need a cleaner on the Mojo. If you run it correctly and with a well sorted system, it sounds great and certainly with no deterimental noise, at least compared to the half dozen or so other good dacs I have had connected to my system via usb. If you have noise then look at possible other causes or configuration.
My experience with Jitterbug and Wyrd with USB in any of my systems has been poor. Transients reduced, duller sound. As for USB in a PC. I have tested and reviewed a half dozen usb bridges and still prefer straight USB to a converted USB to spdif signal. It is relatively easy to get usb sorted if you know what to do and when you buy a dac that does not have a sh***y USB implementation.
Other people have tried the Jitterbug on USB and found they didn't like it. Stories like collapsed sound stage, duller sound, etc. However the JItterbug can not destroy sound, as it does not damage the music samples. What people hear with the Jitterbug, is the music as it's supposed to be. That is confirmed when we A-B between USB with Jittebug and optical.
What it means is that people liked having the noise from the PC having an effect on the Mojo. I was similar in the first moments when adding the Jitterbug. I was very surprised how obvious the difference was when adding the Jitterbug. However I told myself, that how it's supposed to be. Like all changes, I said to myself, get used to it. I carried on listening and in time I appreciated the new sound and loved the Mojo sound more. It was less bright, or less in my face, with more texture and tonality. When I did an A-B with optical it was identical, so I was happy. I would never run a Mojo without Jitterbug from PC now.
If you disagree with me, that's fine. I have said my peace/piece; whichever you prefer. For more proof, Rob Watts added noise filtering (same as Jitterbug) to the USB on the Hugo 2, and TT2. I can not hear any difference between USB and optical on either of those DACs, and I have both. I don't really use Mojo with my PC anymore, or barely at all ever. I just help out in the thread sometimes. If you disagree then I won't spend any more time on it.
I could use coaxial or optical. What would be better? I'll need to get a soundcard from somewhere but if the quality improves it would be worth it.
You can use Jitterbug with USB or optical. Coaxial would OK, but optical is reference sound, only because it's cleanest. If you need to buy a converter for optical, I am sure it would be fine. In my last PC I had a very cheap converter that was not even a sound card. It attached to a purpose built header on the motherboard, which sent sound bits to the converter. I was worried that with it being such a cheap item bought on ebay, that it would work inefficiently. However it ran flawlessly. I never really used it though, unless I was just comparing my USB sound.
Have found picture of the PC card I used. There was a specific header on the motherboard for this very attachment. As you can see it's a converter from USB to optical and coaxial, and not even a sound card. No DAC chip.
I made sure my new PC build had optical on the motherboard.