best cd player for 300-500 bux

i concur with cleaneduphippy--use your $300 on a used dac like the cambridge. your nad isn't a great player, but it'll do just fine as a transport.

The best move I ever made was to a stand-alone DAC. I have the PS Audio Digital Link III and can run coax digital in from the CDP, Optical digital in from a Mac, and USB in from my laptop. Great sound, flexibility and reliable. On sale here at Audiogon for under $500 and new for just a bit more. I don't have one for sale nor do I know any of the current sellers. It will make a new source out of your old one. I have even used it with an inexpensive Oppo universal player and it is fantastic. Good luck in your quest.

So far used Alesis Master Link ML9600 for near $500 will get you one of the best red book CD playback + recording and possibility of the superior CD24 format you can get the benefits from and finally great headphone output capable to deliver 180mw i.e. drives upto 50Ohm impedance earspeakers without headphone amp(although I'd recommend for the better volume control). It also has digital in-out so you can use it as an interface i.e. DAC as well.
Got one recently for copying my records to CDs (has superior DAC and ADC)... Maybe it's a 'toy' but can't stop enjoying it how it does recordings and playback!

Second for the PS Audio D Link III. At the prices these are going for used it's a true bargain. You need to spend a lot more $ on a cdp to beat it.

Or the MHDT paradesia DAC that seems to get raves anytime it gets brought up. it's on my demo list.

I concur with getting an outboard DAC though. in your price range to get anything that has a chance of being an upgrade, that would be the way to go.