Eeh.. while I have no doubt that there is frequency colored gear out there, I've personally never encountered it outside of a slight roll-off in the 20KHz section.
My personal experience is, that I can't for the life of me hear differences between electronic components (Headphone amps with a large output impedance excluded). My buddy wanted a stereo amp next to his AVR because everyone (not me) told him that AVRs sound bad. So he loaned a 3000€ Streamer/Amp combo from Cambridge audio. Exited he was and let me in on the test. We hooked it up and... well.. pretty long faces on that couch once we played music (though not unexpected from my PoV).
No difference, except very slightly leaner and more focused bass (probably damping factor related). Needless to say, he saved the 3 Kilobucks for sth else.
Yes, generally a flat in room FR will sound bright, thin and is not recommended as the final setting. It is supposed to be the first step before you apply the house curve, which is governed entirely by your own preference.
Most speakers are already tuned to a downward sloping FR, for folks that do not use EQ.
It is so frustrating in my older Denon 4311ci that I'm not allowed to use any tone control if I've used my Audyssey MultiXT32. So, I guess I can perfectly optimized frq resp and time delay, that sounds thin and flat. Ugh. This is my one single biggest reason to want to upgrade to a new AVR