I realized I had confused SINAD measurements with pure signal-to-noise ratio. SINAD is generally about 5 dB lower across the board for all AVRs. Interestingly, Denon AVRs seem to have a gain of around 26–29 dB, which is slightly lower than I expected.Where are you pulling these figures from, precisely? Looking at the reviews I'm not seeing anything over 90-ish dB SINAD at any power level for the 3800H and 4800H.
Also keep in mind that Amir has given the Denons a bit of a handicap here by using "Pure Direct" mode for the measurements. In practice, almost nobody will be using them that way since that disables all processing. When not in that mode, SINAD will decrease a fair amount.
That said, my main point still stands. Achieving 85 dB SINAD at 5 watts (worst case of all Denons) of the integrated amps in AVRs is just really good. This level of performance is great —essentially completely transparent for any listening purposes.
Also, the "pure direct" argument applies to DACs as well. Once you introduce preprocessing like EQ, loudness curves, or room correction, SINAD will likely drop by an amount proportional to the corrections applied (in dB). This is worth considering for anyone prioritizing transparency over processing features.