i was getting a hum from my rel s12 subs, which are hooked up to my SE84UFO2.1. i'm going to attach a jumper cable from the black speaker post to a ground screw on the amp, but which one? i tried the black screw attaching the transformer to the deck of the amp, and it worked, but the screw itself doesn't lift up enough to allow me to insert the bare wire from the jumper securely. (perhaps there's a nut underneath?)

which screw should i be trying to attach the jumper cable to?

thanks