On 2020 models as long as you are externally powering all of your speakers which it seems you are all you have to do initially is select pre amplifier amp assign mode then connect up all 7 speakers to their respective binding posts on the external amp then RCAs from the external amp to the receivers pre outs and run Audyssey . No need to do anything different afterwards to fully disconnect all externally powered speakers from their respective internal amps. Also changing amp assigns after running Audyssey disables Audyssey until setup is rerun again.x3700h owner. Wanted to post this for others. I kept reading PreAmp is all or nothing and there was confusion on how to get a configuration other than 11.2 pre-amp mode. When you run the initial setup config wizard it asks you what speaker configuration your running. I have 5.1.2 (5 floor, 1 sub, 2 middle height). After running Audyssey I saved the configuration and went to the Amp Mode setting. Now after switching on Pre-Amp mode it displays the 5.1.2 speaker configuration that I previously setup. I could here the Amp shut off and am running a Outlaw 7000 amp. Flawless! Now when I add more speakers and another amp I'll run the initial setup wizard again. I searched all over for a clear explanation from Denon or Audioholics and there was no info on speaker configs with pre-amp. Hope this helps someone.
EDIT: There's also a way to just disconnect the fronts internal amps like this in 2020 and older models when running less than an 11 channel setup and at least externally powering the front 2 mains. You simply select 11 channel amp assign mode but before running Audyssey you go into the speaker settings and turn off all the channels your setup doesn't use then run Audyssey.