The difference between the 6xx and 7xx series Onkyos has always been the addition of pre-amp outputs, THX certification and associated THX processing modes, extra conveniences like IR inputs, and slightly more rated power for the 7 series.
If you don't intend to use an external amp then you don't need the pre-outs and while THX certification is nice, power wise there isn't much difference.
Every year they seem to migrate features from the higher models down to the lower models and the 607 now has pretty much everything most people need in a receiver. Given that it has been out for some time now they are selling new at half the original msrp. I think the 607 is the current sweet spot for price vs performance in the Onkyo line up.