Warm relaxed digital coax

I have not experienced huge difference between coax cables. So I’m not completely sure you’re on the right track. Cable manufacturers that are known to produce warm sounding cables are:

Audience, Audioquest, Acoustic Zen, Cardas, Kubala Sosna  

In my opinion with Kubsla Sosna cables in the system you should not experience any brightness. If you do, digital cable is probably the last place you should look at as a culprit. I would look into room acoustics and or different warmer sounding speakers. Just my $0.02

Is there a reason why you can’t use USB with the dac?

Because if you can, there’s a really nice Audience AU24SE+ USB cable on USAM right now. 
https://www.usaudiomart.com/details/650117580-audience-au24-se-dual-wire-usb-one-meter/