To elaborate on this, Denon receivers support 2 speaker configuration presets. The presets save and contain all speaker settings and individual Audyssey profiles. You can switch between them with the push of a button. In my media room, I have:
Preset 1: Movies: Front LR amps off, all other amps on, 7 speakers with 2 subs configured with Audyssey
Preset 2: Music: All amps off (preamp mode), 2 speakers with 2 subs configured with Audyssey
The front LR are always powered by a Nova 300.
I use the AVR's amps for center and surround duties in preset 1 for movies and TV.
The AVR is at its cleanest in preset 2, when I need it to be. Since you have 5 channels of external amplification, you can do the same thing for your 5.1 setup.
We talk about SOTA engineering on this forum, but some level below that is acceptable to human hearing in a typical domestic environment. I can't put a number on that, but I find Denon AVRs to be at least as good as my hearing. I have a Schiit Modius with 113dB SINAD sitting next to the AVR, which allows me to bypass the AVR and use a laptop to feed Dirac processed music directly into the Nova 300. The only difference I can hear (with subs turned off) is the difference between Audyssey and Dirac. One does not sound better than the other to me from a hardware perspective.