Any experience with this?
Yes, I know that the 81x at 9V is said to be similar to running an 81 at 18V. I have an 81x on the way, and said "what the hell, it's worth a shot." I have two EMG-loaded guitars currently that are both 18V.
I did some searching and hadn't found reference to anyone running an X at 18V.
I'm kinda hoping someone here has tried it.
From everything I have heard running the X's at 18v makes them sound worse. I haven't tried with my own.