While I am completely unfamiliar with those model #s, I think you should sit tight. Just wait till you get home. This stuff only gets better and cheaper. The SPDIF is just fine. Not until you get a BD player would I even consider changing out. M-ch analogs will work then, but you will have to research cuts to the LFE. Bumping up the sub gain will not work correctly, because all of the re-routed/summed-bass from other channels will be disproportionately raised.
So that means researching the players themselves. Also, some receivers have LFE boosters, or so I've read. You will also not be able to matrix 5.1 analogs into 7.1 to the best of my knowledge.
Lastly, one rarely ever sees someone downgrade from a Denon 5xxx series. Denon-philes seem, to me at least, to rather keep an older 5 than a new 4, or an old 4 over a new 3. I've never owned a Denon receiver.
Good luck.