I suggest you reduce audio level for headphone output in ALSA to increase the useable range of PulseAudio controls
Use alsamixer in terminal to adjust audio hardware settings
Use F6 to find audio card for analog audio
Then find control for headphone output and turn it down to around 20-30%
And use this command to save new ALSA settings
sudo alsactl store
pulseaudio equalizer is introducing audible overdrive (+4db on bass, -2db on mid)
If equalizer is boosting bass by +4dB, audio level before the equalizer must be reduced by -4dB to maintain a level below distortion threshold
Audio controls in PulseAudio can have a maximum level of 150% and any level over 100% can cause distortion on some systems
PulseAudio default is usually CD quality (16bit 44.1kHz) for backward-compatibility with older systems
see this post for how to reconfigure PulseAudio
How to change audio bitrate? - #3 by nikgnomic