What’s the best sound app to run on my iMac?

Are you looking for an app to playback or create sound?

The native OS X App for playback is iTunes.

The native OS X app for creating is GarageBand.

For third party playback and creating Apps,

https://www.aimersoft.com/manage-music/music-players-for-mac-el-capitan.html

https://www.macworld.com/article/668280/best-music-software-for-mac.html

I'd say Audacity...

https://www.audacityteam.org

Pro Tools 2018.12...

https://avid.secure.force.com/pkb/articles/compatibility/Pro-Tools-Operating-System-Compatibility-Chart

https://www.pro-tools-expert.com/home-page/2015/10/1/mac-osx-1011-el-capitan-compatibility-update-compilation

You're welcome.

Can't remember for sure, but I'm pretty sure that iTunes in El Capitan had a built-in EQ with sliders.

see > Adjust the sound quality in Music on Mac - Apple Support

If not, check out > https://eqmac.app/

or > https://eqmac2.en.softonic.com/mac?

VLC player?