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 (Carnhill) as well as a discrete driver circuit. Of course the entire filtering section is passive and mirrors analogue equalizers from the 60s. There is separate frequency selector for low boost/cut sections. And you get the true bypass switch as a bonus!