Jordan, welcome back. Yes, there's a standout receiver in the $500 price range; it's the Onkyo 709 which Tom already mentioned, and the factory refurb is available for about $530 from
Accessories4less . This has the major advantage over the 609 in having the much superior Audyssey MultEQ XT, while the 2EQ in the 609, among other factors, doesn't do the important job of equalizing the subwoofer channel.
As to the difference in the three and four digit Denon models, they're identical, but the fours were intended to be sold in somewhat higher end outlets. The 791 is identical to the 1911. The three digit labels have now been dropped, so there is no 792 corresponding to the 1912.
