Remotecentral.com has Denon serial commands, I think. AVS forum member Batpig has a Denon AVR website that is full of info. You'd use a PC terminal emulation program to troubleshoot serial commands to the Denon. I don't know how to send serial commands from the Elk, thought I saw mention of an SDK somewhere around here.
Or, you can use a 3rd party solution, e.g. CQC, HomeSeer, Elve, MainLobby, Premise (it's free) to stand between the Elk and Denon. If you're going to spend money on one of these, make sure someone else has done what you would like to do already (e.g. control their 3312), successfully. Using one of these platforms, from a touchscreen, you could control the M1 as well as the Denon.
EventGhost, running on a PC, might be another option for Denon control.
How do you want to control the Denon? From a touchscreen? Elk keypad? Remote control (Denon comes with one.

)? Android tab? iTouch?