Rhythmbox is the default player in Linux Mint 20.1. But it still does not have an equalizer.
The best solution is to add Pulse Effects to the whole system.
In the terminbal, simple type:
sudo apt install -y pulseeffects lsp-plugins
It will ask for your password, then press enter.
Search for PulseEffects in the menu. Go to equalizer, enable it.
Click on tool icon. Choose a preset you like, or customize one yourself.
I listen to relaxing Zen meditation music so I chose “soft.”