took the 6600hz slider on my Mark IV and pushed it all the way down. Sounded very dull in the room, but put my ear next to the speaker where the mic would be and it was much a much more pleasant sound. Oddly enough, I plugged in my EMG85 guitar and it sounded 1000x better and I actually enjoyed playing it for once.
I could also hear myself sing a hell of a lot easier without all that high end shit piercing my ears. I think LP'ing so low wont give you a super aggressive metal mix, but it will give you a great round tone for stuff with mostly singing. It also helps with letting a mix not kill your ears when you really crank the volume; like on a car's sound system.