Turn on thump, which I don’t think is as common as it was back in the day, but obviously can still happen.
I would power the entire system on, turn the volume to zero, then carefully disconnect the remote wire at the amp (or at the head unit, at any rate disconnect the remote wire to turn just the amp off), allow the amp a minute or two to power down, and then reconnect the remote wire to see if it pops when the amp powers up by itself. Everything else is on and settled, this is to see if the amp is the problem. Since you unplugged the RCA cables at the amp, I am leaning towards an amp issue as it should not pop with no input connected. Remember, the system is still fully powered on during this test, and shorting the remote wire may damage something, so if you can’t do that carefully, don’t do it as you might damage the head unit, or even the amp if you let it short out.
But yeah, annoying and should not be popping at turn on.