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Denon's manual for the 2310CI, and look on page 3 where it says Cautions on Installation. Denon no longer specifies how much space, but they do say "Do not install this unit in a confined space, such as a bookcase or similar enclosure."
If you leave off the back panel as you said above, and leave the glass door open while its on, you should be fine. The idea is to avoid trapping the heat.