DAC upgrade for a Quad CD-S?


Any opinions on what level DAC I would need to upgrade my Quad CD-S (yes S, not P) CD player? The reason I ask is I would like to get a Wadio 170i or similar to play my itunes/iphone music, but the question is do I buy an inexpensive dac just for the itransport or one that would upgrade my CD player. Thanks,