Is your MM pickup stock (PBG) or an aftermarket? MM's usually have their coils wired in parallel (but that image suggests it's currently linked in series). Series should give you a darker, fatter and louder sound, but won't have the same bite and articulation. If you want the parallel/series option you can easily add a mini-toggle, or (depending on what value it is) replace the pre-amp volume pot with a push/pull to act as the switch.