Well, the L2500 has a passive mode with passive tone control, so if you don't like what that sounds like, then perhaps the L2500 isn't for you.
I'm not trying to be an @$$hole, but what you would like to do with an L2500 by installing a P-style pickup sounds counter-intuitive to what an L2500 is all about....
What is it about the L2500 that you DO like?
Thank you for your two useful comments, 'eastcoasteddie'.
Well, what I like above everything in the L-2500 and the M-2500 is the G&L 5-string neck. There is no better fiver neck IMHO, with its 1.75" nut, it is the easiest one to play, the most comfortable bolt-on 5-string neck on the market. But I don't like very much the tone of the MFD humbuckers, active or passive. I had had an ASAT bass (active-passive switch) and a M-2500 (active only), now both sold. Even with the K-switch (single coil mode) on the M-2500, I - and everyone in my band - didn't like the tone of that bass, compared to my usual other passive basses. I know the L-1005 also has a MFD humbucker.
And I love the vintage tone of the passive AlNiCo split pickup of the LB-100, and also that of the passive single coil MFD pickups on my USA SB-2. But G&L don't make 5-string versions of these.
And I - and everyone in my band - like the tone of my AVRI '62 PB, but Fender don't make fivers like these, and I don't like Fender fiver necks.
So you see what I like and what I don't. I am therefore hoping G&L will issue five-string versions of the SB-2/SB-1 or of the LB-100, if they ever exist.
Meantime, I am asking what kind of tone a L-2500 with a vintage tone pickup in the neck position would yield.
I am excited seeing a couple of '98-'99 L-2500 Custom (with the 4+1 headstock) available near my place, I may very soon buy one of them.
And once with the single-coil mode switch installed, in passive mode, if I may get the sound I am hoping for (which seems possible given your encouraging second comment), there will be no need to change the pickups any longer.
Note - 3 of my basses are strung with Chromes (ECB80 40-95 or ECB81 45-100) and one with Rotosound Jazz 77 flats.