Another possibility is that your CD player might not be flat. Marantz, Yamaha and other manufacturers add their slight sound signatures to their higher and models.

I never benchmarked my Yamaha CD-S300 to other devices, but its flyers and web site never says it's completely flat on the analog outputs. The reviews of the player notes that it has the distinct Yamaha signature too.

Also "Monitor" and "Flat" devices as their name implies sounds pretty flat and dull.

All-in-all you should consider taking response graphs of your devices rightmark.