pickups to pair with the spb3 on the bridge too increase mids?

Is it a PJ bass? Or are you making a new bass (or having one made)?

 

I'd look at either STK-J2b, or the SJB-2b. The SJB is a single coil, the STK is hum-cancelling.

 

One way to increase mids (and output) is to wire the two pickups in series.

 

Ah, in that case, since you have to route a new pickup hole anyway, I'd be tempted to get a set of SJB-3 quarter pound J pickups, route the holes for them about an inch apart, and use them as a humbucker in the bridge position. That's something I would do.

Admittedly, I'm an odd bird. But bass players seem to exist in two broad categories -- traditionalists and boundary-pushers. A traditionalist would probably keep the P bass as a P bass and put a pickup in it that gets a vintage sound. You're converting a P to a PJ and you're using a P pickup most traditionalists don't like, so maybe you're up for something way outside the box.

I really like unique basses. Doing that would give you a bridge pickup that could definitely keep up with a P pickup that most pickups would struggle to keep up with. It would allow a flexible single-coil tone in those situations where noise isn't an issue depending on which pickup you want to use, and in those situations where noise is a problem, you'd still have a usable bridge pickup. Plus it would look cool.