Just bear in mind there is no 7.1, all it does is just produce the same mono EX/ES signal out of the two surround back speakers.
When you switch ON the surround back speakers the amp will use matrix processing regardless of whether the track is 5.1 or 5.1 EX, DTS ES Matrix. A DTS ES Discrete encoded movie (true 6.1) will automatically playback in this format if the surround back speakers are switched ON.
The Denon will also give you the option of MATRIX OFF when you have the surround back speakers switched ON (just press the 6.1/7.1 button twice). All this does is just produce the surround left info out of the surround back left (same for surround right) simultaneously i.e NO EX/ES decoding.