How to: audio equalizer
and [dynamic range compressor](https://en.wikipedia.org/wiki/Dynamic_range_compression)

can also boost the volume in general, or increase/decrease stereo width (like [crossfeed](https://www.foobar2000.org/components/view/foo_dsp_meiercf) just worse)

has the convenient side-effect of reducing the pause between songs, so gapless albums play better with the eq enabled (just make it flat)

not available on iPhones / iPads because AudioContext currently breaks background audio playback on iOS (15.7.8)