My guess is, nothing in terms of audible sound quality, other than they tend to have inferior components relative to the 8000 series. If they would save a few cents to use the AK4458 instead the AK4490 in the 8000 series and the top Denon AVR models, you can expect they might have taken the same approach to other components. The DAC choice to me is just a tell tale thing, but I know few people would take notice of such little difference and read into it further. It is not just a one off thing either, in the older models, such as my AV8801 vs AV7701/02 and the Denon AVR-4520, you will find the same kind of difference.
That's why imo, the top D AVR models are better deals than the 2nd from the top M AVP/Amp combo. If you look at available bench test results, the top D AVR amps also showed better results than the Marantz MM7000 and even the MM8000 series amps.
I am not recommending the 8000 series either, and would only consider one of the previous year model at deep discounts if it comes with the full 3 year manufacturer warranty.