cable as tone control

I'm looking for recommendations into a cable that can help me control breightness that I cannot otherwise tame.

The system is actually my portable - A Panasonic 580 "walkman" type CD player, Grado RA-1 headphone amp and Koss portable headphones. This is a Headroom recommended system and I use it while driving my '66 MGB, in which I dare not put an aftermarket stereo. And it actually sounds quite good considering the source player.

What's your recommendation for a cable that will help tame the brighness as best I can, since I can't use a DAC or bring my SCD-1 in the car! (The Grado RA-1 headphone amp uses standard RCA cables to connect to the Panasionic with a mini stereo adapter).

I do a lot of listening on this system, so I'd look at cables with a list price of up to $1500 (i.e Golden Cross, Quatro Fil, etc. -- but my gut feeling is that's there is some lower price stuff out there that might be specifically good at helping to tame that digital edginess that I hate so much. --Lorne