If you search electric mistress in youtube you’ll find a video by some british guy who has demoed a lot of vintage EHX pedals where prior to the demo he shows you how to overcome the volume drop. It’s basically installing a capacitor and resistor in parallel with some existing ones on the back of the circuitboard (i.e the side facing you when you open ip the pedal). Looks dead easy if you have some soldering experience.

My original vintage mistress had a huge volume drop, but I’ve recently picked up another on ebay that looks to be a later version – it has a green circuit board and dashes in the circles around the knobs. I also noticed it had a capacitor in parallel off the IC chip in a similar place to where british dude has modded his unit. Will report back if EHX corrected the drop in the later units.