If I can follow up on this, these sound like important, but not earth-shattering, differences.These differences make ALL the difference if you're looking for resolving, high-end CD playback that aspires to reproduce the sound of real music in your room.
In other words, it sounds like the 3910 is a decent player to begin with, just one that can be bettered on its redbook playback at its cost point by a dedicated CDP. The rest of the system isn't a super-high resolution setup anyway (Audio Research SP-6A preamp, Quicksilver KT88 amps, Thiel CS1.2 speakers), so maybe the Denon would work OK there.
Your definition of decent may or may not correspond with mine, or others', but it's no matter. To me, it sounds as though you're looking for something that will provide pleasant music and need not meet the definition of high-end audio. If this is the case, and if the Denon meets other requirements such as video and multi-channel playback, then the Denon 3910 is an excellent choice. If high-end CD playback is the primary purpose, then you can do better in a dedicated CD player, IMO.
As you mention, the resolution of any digital player will be less or more apparent depending on the resolving power of the rest of the components in the system. Even here, your experience and satisfaction will likely differ from the experience of others, and what others report will be more valuable if you have heard the components that comprise their systems.