Warm relaxed digital coax

I am a big fan of the Audio Research I-50 it is a great amp with lots of midrange bloom and musicality. That great tonal balance is coming from the I-50… you will not loose it unless you go to some very lean interconnect. I use Transparent with my Audio Research stuff. I find it communicates the details without tipping the tonal balance a bit. If you want to explore an alternative, I would give DHLabs a try… make sure to go as high high up in their lineup as you can. 

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

A stock Forte 4 and AR i50 is not resolving enough to easily reveal differences in digital coaxes.

But, you could try an Ansuz cable, which could sound a bit more detailed (after pulling teeth) and maybe offer a slightly different tonal balance in a more resolving rig+room. A GR research upgrade for your Forte could make it more resolving, if you’re a diy kinda guy..