I've had all 3 Onkyo systems--I had the 580 only long enough to return it for credit toward the 680. I had the 680 only long enough to return it for a refund. The difference the 780 provided was stunning. The 580 had "thin" sound, and not just b/c of the small main/surround speakers--the subwoofer was not only thin sounding but boomy, too. The 680 had 6.1 but still had the small, thin sounding main/surround speakers, although it did have a powered subwoofer rather than the passive sub.
The 780 had exponentially better main/surround speakers than the 580/680 (not to mention the speakers had 3 db more sensitivity, which when combined with the more powerful receiver went even further in creating a more convincing sound field) and the sub has great sound. There are significant connectivity issues regarding the 3 receivers, too, with the 580 offering much less than either the 680 or the 780.
I understand the dilema here--I sourced the 580 at $150 from Fry Electronics, the 680 at $280 from Fry's, and was reluctant to give up the great deals to get the 780. I ordered the 780 from Onklo's online store, refurbished (minor cosmetic defects??? I can't find them!) at $350. A week later Fry Electronics had the 780 on sale for less than $300.
Sometimes patience can be more than just a virtue. Now I wish I had had more of it