Its hard to say. I like the Marantz sound. Marantz typically has a house sound that leans on the warmer side. A warmer sound is more forgiving with bright sounding Cd's, especially older recordings and rock. The power gain of 5 more watts is negligible. I have never heard the Denon. But there may be some synergy of the Marantz receiver and the Marantz CD player and the two may compliment each other. Based on the review below that seems to be the case. So it may be a reasonable upgrade.

Here is a good review on the Marantz-

Marantz PM6004 review from the experts at whathifi.com