I can tell you that I thought the MEC and Basslines pickups were both very similar. IMO, the pickup choice will color your overall tone more than a preamp swap will, if you don't use very excessive EQ cuts or boosts. Unfortunately, the only real way to find out is to buy diff pickups, try em out, and sell 'em if you don't like 'em! It's a drag, I know. But it does seem like you're doing your research.
I feel like the MECs and the Basslines are also pretty hot with an active preamp, and I feel like I get a pretty modern tone out of my passive pickups with active preamp, FWIW. But I actually feel like the MECs are pretty good pickups, and a lot of folks over at Warwick's forum who have gone through this tend to agree. Try changing the pre first if you really wanna change something, and then change the PUs later if you still feel like you haven't nailed it!