I am NOT a pickup expert by any stretch, but I'd say that my MK1s sound much better with my Bart preamp installed. This is on a 2004ish SR505. I've heard before that the preamp is the weak link, and that the pickups are fine. In my (very limited) experience, that is the case. The new preamp also has a 3 way mid switch (which my model didn't have originally, but later ones do), and I really like that. I can basically set my bass's tone controls how I like, and then switch mids to go from brighter hi mids, fairly even "mid" mids, and lower mids being the focus. It's pretty great for a quick tone switch.
I've never installed after market pickups in a bass, and this is the only preamp I've changed as well. All that being said, the improvement of just the preamp has made me wonder if I should also do the pickups and really see how this bass can shine... But so far, I really don't have a need to.
Anyway, the clean tone on the modded SR505 is FAR better than my stock SRX505, which is why I'm looking into an Audere preamp for my SRX. Maybe Bartolini as well. Besides already having a positive experience with Bartolini, it has the added positives of having a master volume built in to the preamp, so I could match the volume of my two main basses. This is important to me, since I use a lot of dirt etc. and want similar output on basses so I don't have to reconfigure pedals etc. all the time. I want to be able to just crank the volume on both (as I typically do) and just go. It also helps that the Bartolini has a push/pull pot for active/passive. The Audere doesn't have a master volume control, nor a push/pull for active/passive. Of course, the Bartolini is roughly 60% more expensive for a similar 2 band setup...