Rockbox seems to take pretty much any device it runs on, in my experience, to a whole new level and despite the software being almost 20 years old (from its initial release), it still outperforms nearly every other piece of firmware or OS I've tried in terms of sound quality. It's possible that part of the reason is because I live in Europe (and until Brexit happened, the EU), where most built in firmware includes volume limitors to comply with an unenforcable EU directive, which Rockbox overrides. Beyond volume, Rockbox allows you to alter spacial settings, the oversampling rate, the bass/treble intensity, the track speed and more, and it even has a karaoke mode which strips out centre panned audio on stereo tracks. Very few device manufacturers include these options in their stock firmware.
I guess that proprietary manufacturers are more concerned about having to abide by things such as the EU's headphone volume directive and various DRM restrictions, which Rockbox doesn't need to worry about as it's developed on a non-commercial basis.