on a 2910 you need to use the analog outs.
3910 has hdmi (audio for DVD-V or 24/48 DVD-A) and D.Link (digital PCM for DVD-A, DSD for SACD, etc) - to get these to work you need to update to the latest firmware - note that it does support DSD and PCM over Firewire on older firmwares
Nothing is passed on the optical or coax outs on these units above 16/48 from what I remember
I have a 3910 that I grabbed at an estate sale and run it analog 5.1 to a marantz reciever without issue for SACD and DVD-Audio stereo and multichannel
I ran D'Link to a 3808ci beforehand without issue
I am going to grab a Marantz UD5007 at somepoint - this unit supports multichannel DSD SACD and multichannel DVD-A (high bitrate) over HDMI that would mate nicely with my Marantz reciever - used they sell in the $150-300 range