I put a set of Bartolinis in my Mustang PJ and replaced the pickup selector switch with a blend knob. I love this little bass and I can dial in a little jazz to the P sound for variation.
I also replaced the switch with a blend knob, but put in the DiMarzios. I like the sound much better than stock.
I wondered if the blend knob might look weird, but I think it looks fine.
And it sure works a lot better without the pickups cutting out with no warning.