Pultec style EQ in your 500 series rack.
 
Rubber Bands 500 is a passive equalizer absurdly reduced in size. This tiny module is equipped with transformer balancing on input and output (IGS Custom) as well as a discrete driver circuit. Of course, the entire filtering section is passive and mirrors analog equalizers from the 60s. There is a separate frequency selector for low boost/cut sections.

And you get the true bypass switch as a bonus!

 
It is all about these bands!