There’s no device you can put “in the middle” to equalize anything. The player of the music is already on the speaker, it just grabs the data from where it is told to, and plays that data.

Youd need to modify the stored data in some fashion. 

There are settings to control the sub in the app too, as well as an equaliser. 

Thanks for the reply - so bummer, stuck with the sound as is. Even with the Sub speaker there is very little bass / low end. Happy holidays. 

Or you can adjust the equaliser in the app and sub settings. Also there is trueplay tuning.