Yes, the P pickups are the loudest ones we have made. The pickups are not identical through the years. We've tried a few different variations on the number of turns, and used the trim pot to adjust the gain. So it isn't terribly suprising that basses of different ages might have slightly different outputs. However, comparing the split style pickup to the MXY isn't really useful, they are constructed entirely differently, so they are practically guaranteed to sound different. Also, notice the pickup position varies between the two basses.
The two basses you have are two completely different beasts. The most significant difference is the neck construction: set neck on the Epic, neck through on the Persuader. You should be able to hear this without even plugging the instruments in. The construction affects the vibration of the string, so you will hear this acoustically.
You might want to measure from the nut to the 12th fret and double it to confirm the scale length. Most Persuaders were 32 medium scale, and this can also influence the sound compared to the 34 scale Epic.
As a neck through bass, the Persuader would be a more expensive instrument than today's Epic. If you ordered a new Persuader (still available for special order), it would be nearly identical to the one you have presently.
If you want to try an experiment, unplug the filter module from your Persuader and swap it with the bass and treble module in the Epic (it's easy - they are all on modular connectors). You'll have an open hole on the Epic, but you can see how the filter differs from the bass/treble controls. Likewise on the Persuader, you'll have to leave the cover off to access the extra control, but for the purpose of comparing, and learning how the instruments differ from eachother, I think it would be a useful excercise. Then just plug them back the way they were.
Also, another thought to keep in mind is that a particular instrument may sound particularly good for no particular reason. I mean thay they always turn out excellent, but sometimes they turn out extra-good, and we can't explain why, and usually just guess it's the certain wood or wood combination that is influencing it so much.
In any event, it sounds like you're really happy with the Persuader, so I hope you continue to enjoy it for years to come.