There will most likely not be a difference in audio quality, but you will have more features. AV receivers have mostly been the same amplifier wise for the past 30 years, just with differing channel amounts and wattages.

But feature wise, they vastly improve even over a few years. A basic 5.1 or 7.1 modern receiver will have a bunch of 4k HDMI inputs, support for the latest surround formats, auto room correction, WiFi, Bluetooth, and further up the scale there’s Dolby Atmos much more.

So basically, a new receiver will sound the same as your current receiver, but you will get a lot more modern features with a new receiver.

An effects processor with some reverb, flanger, and phaser.

My goal is to collect as many cool pieces of audio equipment and to blast my favorite songs.

To me, a flat signal is unbearable. Sound like the speakers are inside of a wet paper bag. A good boost in bass and some treble boost always makes speakers sound better, I will die on that hill.

You buy an amp for your use to do as you wish.

I have a GX909ES, one of the best receivers I own.

$20 is a good deal for some Polks, idk what he’s on about

I have exclusively used iPhones my entire life. I don’t know what you mean by “restricting to a streaming service”, you can play FLACs on iPhone with a 3rd party app just fine. Just be sure to get one with enough storage for your library.