CD - upgrade, replace used, buy new?

The DAC chip in your CD player PCM63 is one of the finest DAC chips ever produced. It is 20bit but will hold its own against 24bit chips. You could always talk to Stan Warren about your player; whether is it worth upgrading. He is always helpful. Stan will not sell you anything; just give you his unbiased opinion.

Couple of comments on what's above.
The Aiwa XC37M cd changer is discontinued. You may be able to find some closeouts somewhere. You should also check with Stan Warren to make sure he is still modifying them.
It is a very good cd transport for its low cost, but must be modified by Stan Warren to reach its full potential. It is a bit more refined than a Cal Delta (I have owned a Delta and currently own the modified Aiwa).
I would think you would have to get the MSB modified and pair it with a modified Aiwa to achieve the magic mentioned above. I have tried a stock MSB Link I (with no upgrades) and found it to be light in the bass and lean in the midrange compared to a Micromega Duo BS2.

Hey, I have the EAD U2000. I picked it up for 1000 and it just killed the Musical Fidelity A3 CD player that I had before it. RE Marakanetz's comments about the DSP7000, you'd probably be very happy with the sound of this DAC. But like the others said, the EVS has the newest stuff for cheap. I would get the EVS, keep the Pioneer.

There are new old stock Aiwa XC-37M on eBay for auction.

I have to disagree somewhat regarding some of the above comments on the PD-93. Yes, this is a beautifully made machine, but I find its sonics lacking in many regards. I bought 3 of these units brand new at quite a discount, sold one to my brother and another to a friend, neither of which are audiophiles. It was a great value for them both. The third unit I sent to Stan Warren for the full mod. I was still less than impressed, but it certainly was not offensive in any respect. About that time I bought a new Pioneer Elite CLD-79 LD player for my new surround sound system. This had the new Legato Link and I wanted to try it in my main system to see how it compared to the modified Stan Warren PD-93. There was really no comparison, the CLD-79 was a vast improvement in all ways. Fortunately I was able to sell the PD-93 at a profit and move on. Since then I have owned several different CD players or DACs including Meridian, Sony ES, Accuphase, Adcom, and am currently on my second Audio Logic tube DAC. The Audio Logic is easily one of the finest DACs available at any price. After selling the first one I just couldn't find anything as good including the Accuphase DP-65. I use it with a CEC TL2 transport and Genesis Digital Lens. I bought all of this equipment used and have $2700 invested (not bad for a $10,000 retail system...thanks Audiogon and the depreciating value of digital seperates). Getting back to the original querry, the best CD player value I've had in my system (and very much in your price range) was the Adcom GCD-750. Great build quality and you will have to spend a lot of money to beat this player. It was closer to the Accuphase DP-65 than I could believe considering the difference in price of close to $5000 retail. I don't see these for sale often on Audiogon and I suspect it is because they are keepers. Worth waiting for though and I think you can pick them up for $6-700. There are other recommendations I could make, but this one stands out clearly in my mind. There are some excellent reviews on this player if you look around. Good luck.