My first reaction is to tell you to save up until you can get something better and not to expect great sound quality from a $500 pre-pro (or even much higher ones for that matter).
Having said that, Every time a new pre-pro comes out the manufacturer claims big increases in sound quality, when really it's mostly new features (room correction, video handling, atmos, etc.) and pre-pros tend to lose their value faster than most audio gear so you can find some bargains on some decent gear if you don't have to have the latest and greatest.
In your price range for used, I'd try to pick up an Onkyo Integra DTC-9.8. I'm not sure if it has all the features you want, and I'm not sure why you want to run the Oppo directly to your amp. The pre-pro has balanced inputs and 7.1 analog inputs. You can set it up so that you're using the DACs in the Oppo if that's what you're after. There are a lot of reviews about this unit and a few of them for sale under your budget.
Having said that, Every time a new pre-pro comes out the manufacturer claims big increases in sound quality, when really it's mostly new features (room correction, video handling, atmos, etc.) and pre-pros tend to lose their value faster than most audio gear so you can find some bargains on some decent gear if you don't have to have the latest and greatest.
In your price range for used, I'd try to pick up an Onkyo Integra DTC-9.8. I'm not sure if it has all the features you want, and I'm not sure why you want to run the Oppo directly to your amp. The pre-pro has balanced inputs and 7.1 analog inputs. You can set it up so that you're using the DACs in the Oppo if that's what you're after. There are a lot of reviews about this unit and a few of them for sale under your budget.