CD player recommendations for...

I'm putting together a system for a buddy or mine and I need to recommendations for cd players? He has Paradigm Studio 100 speakers and recently picked up a Musical Fidelity A5 integrated amp. Budget is $500. 
Well, obviosly there are many choices at that price. NAD is excellent new retail at about $500. I currently have a Rega Apollo-R, wonderful CDP. You may be able to find a used one at your budget.
+1 on Rega Apollo. I own one and really think the sound is neutral, extended range and the tonality of instruments seems accurate. I have the original and it has a buffer that reads and checks the CD before you hear it.
I think some of the older Sony models like sony 307esd or cdp970, ones with Philips TDA1541 chip, sound a lot better than more current models in the $500 range. They cost less and are built really well.
Just purchased a Marantz CD 6006-very decent not "high end" but surpasses my 20 year old Sony ES carousel player.You should be able to get one for $450...
this might help;
all right around $500-$600
Marantz 5005, good mid to top end sound, good balance, great with system that lacks top end extension.

nad, 546bee, soulful sound, takes the edge off, good for a system with to much top end or to bright.  great with mallard tubes.

oppo 203, good all around sound, good balance, matches well with and plays everything, seems to like amperex tubes