Dolby Digital is definitely supported by your
Denon as well as the other sound formats.
You may want to check that the Denon
(While playing Apple TV) is set to 'auto'.
Your ATV3 is set correctly.
You connected your ATV3 correctly to the Denon.
Certain media channels on the ATV3 output
Dolby Digital and MOST only support 2 channel.
Examples of Dolby Digital content are:
Any movie purchased from Apple,Netflix,HBO Go,
Vevo,YouTube are only 2channel.
On your Denon the are various listening modes
That will 'simulate' surround sound...try them out.
(Main)-Marantz 6012*Outlaw Audio M2200*Rythmik F12 subs*Paradigm Signature S6,C1,S1
******Harman Kardon DMC1000 (music)-Assassin HTPC (movies).
(Bedroom)-Marantz SR5003*Paradigm Studio 40,SE Center,SE One*SVS SB-2000 subs*Sony S6200
(Music room)-Outlaw Audio RR2150*Sierra Two ribbon speakers*Rythmik L12 sub*Yamaha CDR-HD1500