I found the problem, I'll post it here because it seems so obscure - the Denon 2910 manual gives few clues of it.
In Setup/Audio Setup, most settings were grayed out, not available, including Audio Channel and Source Direct. And Source Direct was set to ON, not the factory default.
I also found that Setup/Digital Interface/HDMI Audio Setup was set to MULTI(NORMAL), not the default. I changed it back to 2CH, which made the Audio Setup setting available. I went through the Audio Channel settings, and changed the Source Direct to OFF. Now I get Multi Channel through Ext.In. Life is good again.