Disclaimer: I'm far from an audiophile, I just know from personal experience that if your source is low volume and your output is cranked up you get a lot of...distortion? Interference? Artifacts? I don't know, it just sounds staticky and worse.
Mostly noise because you're amplifying it as well, and possibly artifacts because material converted at too low levels (which is the case of old CD rips that weren't normalized before encoding) wouldn't use all the available headroom, therefore losing one bit.
I still listen to it every day because it's highly curated and has tracks unavailable on any service, but quality-wise it definitely feels like I'm plumbing the depths of a DVD collection when everyone else is watching Netflix.
It was really a shame when what.cd went down, that was arguably the most complete archive of recorded music anywhere.