That's very interesting indeed! I was also under the impression that the bridge-biasing was far more pronounced, but clearly it is more or less in Jazz position. I've always thought that basses with a P/J pickup arrangement can cop a kind of "Wal-ish sound", so this makes sense.
But then again we are discussing humbuckers (and very big ones, at that), so they do sense a bigger area of vibrating string, part of which is far closer to the bridge than the Jazz's single coils are. So when compared to something comparable with two humbuckers, like an EBMM Stingray or a G&L L2000, they are quite closer to the bridge. I still agree that the Wal pickups' unique construction has a lot to do with the sound, though.
Because of the coil pairing on individual strings (and then their parallel summation at the preamp input), they sound quite "flat" (or maybe "even" is a better word?) fundamentally and harmonically (to my ears, anyway). So when the preamp filters away these higher frequencies and overtones, you have the opportunity to "re-voice" each pickup individually and then actively blend them for tones which I can only describe as three-dimensional.
All of which goes to say: In my opinion the pickup placement is at least as important as the pickups themselves, the preamp, and the neck construction (but this is only my opinion).
Anyway, sorry for the long post (I really love Wal basses
) and thank you for doing that comparison on the measurements! If there's anything else I can help with, let me know.