If you are good at manual setup (which most people are not), then the automatic setup is a matter of convenience, plus the option of using whatever EQ it has (which some people do not like, while others love it; that should give you a clue about its importance or lack thereof). The Oppo is a great player, and so using it with your old HK seems like a good idea, as long as you don't mind having to set things up manually, and are good at doing it.
As for the price of receivers that have the multichannel analog inputs that you want, it is true that they tend to be expensive, but you can save hundreds on a closeout deal, like this one for $700 delivered:
Yamaha AVENTAGE RX-A1020 7.2-channel home theater receiver with Apple AirPlay® at Crutchfield.com
It is last year's model, which is why it is discounted. If I were buying a receiver at that price point, it is what I would buy.
But it is unnecessary to buy a receiver; going with the Oppo would save you money.
Now, since you bought the Denon, if you already have a BD player and do not really need the Oppo, that is even less expensive. But you give up the great video processing of the Oppo, which, depending on what you run through it, may or may not really matter. It also plays SACDs and DVD-Audio discs, which may or may not matter to you.
The long and the short is, what you should do depends on what you want, and on how much you are willing to pay.
With the Oppo you would have all of the audio formats of BD. And so you would with the Denon. Or with pretty much any other new surround receiver. So the issues are primarily convenience and cost. The room EQ of a new receiver may be something you love, like, dislike, or hate, and so it is very hard to say how much that should influence your decision. I personally would not decide based primarily on that, though I do like the room EQ of my Yamaha.
