Interesting... so the interconnects probably aren't a factor. Guess the older player is just broken in better
I read a few posts talking about other brands of universal players (Pioneer mostly) where there seemed to be a difference in detail when the bass management was turned on in the player since that caused the signals to be resampled at a different rate. I don't know if your player even has bass management or if it affects the signal going out to the L/R RCAs but one more thing to check. Seemed like setting all the speakers to "large" was enough to turn off bass management in the Pioneer 578...
EDIT -- I guess you are gonna have to stay up late one night and swap the cables just to prove they're not a factor. If you haven't already done so, burn a couple of copies of the same CD so you can get the most accurate comparison of sound when A/B'ing.