"HD Audio" is a fairly ambigious term.
This could refer to the multi-channel (or even sstereo) audio tracks on SACD's or DVD-Audio discs.
Or, it could refer to the new audio formats that Blu-Ray and (the late) HD-DVD formats offer.
In the case of the former, I do believe that this decoding is done only in the players themselves. In the latter, a few, very few, top-of-the-line receivers can decode some of the formats.
But, you're in luck! Since your receiver offers 5.1 analog inputs, you should be able to take the 5.1 channels of analog "HD Audio" from the player of your choice and send it directly into your Denon. ...as long as you make sure the players offer this, and most seem to.