I have Googled myself into near exhaustion trying to find an audio equalizer for Linux Mint 18.3. The only advice I've found is for older versions of LM or Ubuntu, none of which work for me.
I recently moved my best stereo speakers (with subwoofer) from a seldom-used XP Pro PC to this LM MATE 18.3 machine, and their sound quality is nowhere near what it was on the XP box! To think that that no equalizer is available --either graphical or command-line-based-- is almost unthinkable!