Re: Should i change my orig. PAF Pro to a Fred?
The FRED came about because DiMarzio was experimenting with the PAF Pro, by raising the mids (probably to get it to throw an amp into overdrive a bit better), and when they did this, a whole bunch of harmonics unexpectedly jumped out. The output of the FRED (305mV) is only 5mV higher than the PAF Pro (300mV) (which is big enough to make a difference between bridge/neck slot, but not that huge), so you won't see that big of an output difference.
The Mo' Joe (320mV), in my experience, actually differs from a FRED. Although they both have nasty (but not overbearing) harmonics, the FRED sears and burns a little more, while the Mo' Joe growls and has... more, if you know what I mean.
What sound are you looking for specifically? That really affects your final decision in pickups, if you tell us what sound you want we can give you a more accurate suggestion. And since you're using a JEM with PAF Pros in it, I'll assume you're talking about a basswood body and a rosewood neck (is it a 77FP?)
And on a side note, shouldn't this go in pickups?