Just my $0.02: I wish I'd gone down the route you're thinking about going down. I had a pair of BigRigs in my previous bass and found the bridge pickup lacked the sort of presence and general heft that I wanted. Stew from Nordstrand recommended the the BigRig/ Big Split combo you mentioned here and I really wish that's what I'd gone with.
Per your comment about wanting a "fat, warm and round" sound, I certainly think you'll get that with the BigRig in the neck position. With my previous instrument, I played with the neck soloed most of the time and I was absolutely in love with the sound (I paired the pickups with some La Bella tapewounds and they worked so well together - definitely in that fat, warm and round zone). The BigRigs have that vintage P-Bass thing going on in spades, but they feel more articulate to my ears, which is positive IMO.