The JHS Haunting Mids is an EQ circuit that controls the mid frequencies along with giving a 10dB clean boost. From the JHS site:
The Mids and Sweep controls work very closely together so you can find the perfect frequencies to boost or scoop mid frequencies. When the Mids knob is at 12 o'clock, no frequencies are boosted or scooped. Turning the Mids knob to the left of 12 o'clock will begin to scoop out midrange frequencies, while turning the Mids knob to the right of 12 o'clock will begin to boost midrange frequencies. The Sweep knob controls the what frequencies will be affected. Sweep knob to the left will cause low-mids to be affected, while Sweep knob to the right will cause hi-mids to be affected. The Lo/Hi toggle is like a "Q" function and controls how sharp the frequency peak is that you’re affecting with the Mids knob. "Lo" is a softer and wider peak for slightly subtler affect as you tweak the Mids/Sweep knobs. "Hi" is a sharper and steeper peak giving you a very pointed and defined effect as you tweak the Mids/Sweep knobs.
Should fit nicely in a 1590B. Schematic for reference.