Well, I'm an honest-to-goodness research scientist (PhD electrical engineer), but what I know has nothing to do with audio.

However, I'm 100% certain that if you have a $200 receiver, $200 speakers and a $100 CD player, if you have $500 to spend, you won't get the best bang for the buck by upgrading your CD player.

(Obviously, the first thing to upgrade is the cables.)