Audio Equalizer for YouTube Music app
I use Viper4Android and it works for me on YouTube (Have been using it since ICS era). It requires root though. 

Installation method various depending on your Android version.
If your device is supported by Magisk, I recommend you to use the Magisk Manager and install the Viper4Android module through it. 
Official way is to install using the app Official XDA thread (You'll probably need to set Selinux mode to permissive to let it install drivers which I personally don't recommend, so go for the Magisk way)