My room sits at 38db right now. Not anywhere near silent. But I will say the ASD XDR appears to not be noticeable. If you're a musician or such where your professional job requires minimal background noise, you might want to do more research. I have a LaCie d2 as a TimeMachine drive and the drive is louder than the XDR even during its more silent moments.
I will say that I plugged the monitor into the power outlet and then my face was near the top of the monitor to start plugging the TB5 cable to my mini. I was surprised to feel the fan was on even though it had no TB5 cable connected. So I'd be curious to do further testing to see if the fan runs when the computer is off.
Bass seems to get a little bit deeper. Biggest thing that I've noticed is max volume though. While I have no scientific measurement, I feel that my old ASD at 100% is closer to ASD XDR at 80%. If I go higher than 80% it starts to hurt my ears. So it seems to have a lot of range.
Yes, I do see vignetting around the edges using the default out of box settings/brightness while viewing this white forum background on MR. Less noticeable on colored area of the screen.