You have more flexibility with a outboard pre and can swap to other units on a whim with no hassle - A Tone Hammer will give you the 'Aguilar sound' in place of routing for and fitting the OBP-3, something like a Basswitch will be a solid studio grade eq with two bands of semi para eq (and a whole heap of other features), Nordstrand do an external pre based on their on board preamp, Alien Audio's Tidal EQ/Preamp pedal is a cracker too etc etc.
Depends on what you want really.