A tube/SS buffer after the CDP and before the Amp would probably do just as well.

It's NOT that simple actually. It needs a true Pre-Amp input. Aux will still go through the Denon pre-amp. If anything you might try a digital out from your CDP (good coax if possible)and use the Denon for the D/A. You'd have to experiment to see which was better.

The tube buffer might work too. In any case, I think either Direct or just plain Stereo mode would give you what you're talking about. Again, though, it WILL be going through it's pre-amp so unless it has a Pre-In don't waste money on adding one.

You need a tube buffer, a tube preamp will be difficult to employ with that receiver.

http://www.pacificvalve.us/YaqinTB.html

http://www.audiodigit.com/?section=81

There are still some Musical Fidelity X-10v3 tube buffers around.

The other somewhat obvious solution is to get a CD player that sounds better and more to your liking. A buffer is still more like a tone control to add a warm euphonic sound to the player. A good player will sound natural and easy without anything needing to be added. Have you looked at Audiogon for a Rega player perhaps? Not bright, and all around very musical sounding. Others are out there of course...Something to think about!