I've tried single lip from time to time, but the vibration of the mouthpiece on my upper teeth is very close to chalk-on-blackboard for me, even with a fairly thick pad.
All of a sudden I have noticed this as a problem, but only with certain mouthpieces (seemingly plastic ones only). A bite pad doesn't help the issue for me either, the vibrations go straight to my brain. It's not comfortable at all. I avoid those mouthpieces, I have slightly crooked front teeth if that has anything to do with it, but that same problem is what keeps me from using double lip more often, as the "point" of my one tooth digs right in to my upper lip, even without any pressure or tension.