I'm told that the P90 is different/special because of it's form factor? From my reading, the size of the P90 - let's them arrange the pickup bits that give a traditional P90 it's raunchy middy bite.
So in a humbucker shape, I'm not sure you'll get the full glory of a P90 (in the same way a humbucker never quite nails a single coil when spilt).
But since you already have a P90 equipped axe - How about a set of filtertron's or gold foil style pickups? I'm quite a fan of filtertron's, a decent set has dynamics for dayyyysss and I like the vintage-y tone running through a slightly gritty tube amp.
I played a Gretch Electromatic (MIC) a while back through a Peavey Classic 30 - that was bluesy heaven. Pleasantly surprised by how good that combo was.