The Denon 2808 does not decode the HD audio formats, so that would knock that one out, for me anyway. The 2309 does decode the HD audio, but is only 100 watts per ch., has 1 less optical input, and 1 less HDMI input than the Onkyo 806.
The Onkyo is more bang for the buck. Denon makes very good stuff, but puts a premium price on them. To get the same features as the Onkyo, you'll pay more for it. Just remember, with the Onkyo, don't close it in. It needs room to "breathe".
. In fact, the 2807 was the first avr that did. But it falls a little short on video processing/up-conversion and has a two faced remote (you have to flip it over to access many of the setup buttons). But it is a Denon so reliability and over-all performance should be above par. For me it would be a tough call between it and the 806 but I'd probobly go with the Onkyo.
Ok, my mistake for not reading the whole thing, at their site. I just looked at all the logos, under the pic. If the receiver decodes the formats, it usually shows the DolbyHD, dtsHD MA logos there, and they aren't there for this receiver: Denon USA | AVR-2808CI Sorry for the confusion..........