It's entirely possible the so-called 'digital brightness' you are hearing is absolutely fine except that it's being clouded and blurred by a lot of digital noise and AC noise that has been left untreated.
It really doesn't make sense to attempt to use cables, ic's, etc. as tone controls. It's not much different than putting a band-aid over a band-aid.
Especially, if you ever upgrade.
I'd recommend the dedicated lines as others have and you might consider auditioning the Foundation Research passive, dedicated, and bi-directional filtering (digital noise is bi-directional) line conditioners.
The LC-1 and LC-2 line-conditioners are small boxes that come with their own built-in power cable and cost about as much as a good power cable, yet do far more. The old versions were considered the best by Marty DeWulf of Bound for Sound in Aug. 2003 issue. The new versions are about 2 or 3 times better than the old.
-IMO