You might want to consider the Cambridge Audio 740c. I owned this as well as the 840c. They sound very similar and are excellent players and perform well above their price point as the 640c does. I did not mention them because I don't think that they would lend your system much benefit given the speakers and amp you have. And if you ever decide to upgrade, any of these Cambridge players will do well in an audiophile grade system. If you need more warmth in your system then go with a Marantz.

For the price, I don't think that you can do much better than a new 640c or 650c. If looking for a used player, I would not buy one more that 8-10 years old due to reliability issues.

And yes, a dedicated CD player most of the time will sound better than a DVD player. If you are looking to upgrade to a Universal Bluray player than consider the Oppo BD-83. I does everything well and even plays SACD's.