I was reading about a recent Jackson Dave Ellefson signature model. Mr E said this   "I went back in time with the P/J setup. The very first Jackson basses that I got back in the day had that configuration, because I noticed that if you flipped the P pickup to the reverse position it got rid of an annoying low-mid frequency that just doesn’t work in a metal setting, because it gets the bass right down inside the kick drum."   Which sounds very much like a good thing; when, th