Hi.
According to this post: https://www.megadrum.info/forums/viewtopic.php?f=3&t=1929#p21063 there should be no actual difference between using mono and stereo cable to connect a hi-hat controller to input 1.
When I connect a hi-hat controller to input 1 with a stereo cable, maximum level reported on the screen is around 440. When I connect it with a mono cable (though connecting the HiHat power source to the ground) maximum level reported on the screen is around 32.
Is this a feature or a bug?