I use the 6-channel outputs from my 2910.
I didn't really notice a difference. Except... On a few CDs, primarily Norah Jones' second album, using my receiver's bass management for the crossover, whether through 2-channel CD analog inputs or over digital, there are spots in songs on this CD where there are "sparkly" artifacts on certain notes. This noise was audible at all volume levels. The noise would go away if I switched the left/right speakers to large, or engaged one of the various sound modes in my receiver. But using the Denon's bass management over the 6-channel analogs (although the signal only goes out over 2), the noise is not there.
Other than that, I haven't listened close enough to notice a difference. But not having that pesky noise means I don't really feel like going back to the CD 2-channel analog or the digital connection - unless I want to engage one of the DPLII Music, Logic 7, or DTS:NEO6 modes.
I also use the 6-channel outputs for all my DVDs, unless it's only in 2-channel and I want to engage DPLII Movie or something.