I did a recording session where I DI'd the bass, and each of the strings was at a totally different volume. I tried to balance this out by adjusting the pickup heights, but I could only get a compromise. Lots of compression was needed to iron it out. Acoustically the string response is very even, so I think this is an issue with the stock pickups. I'm using the La Bella Mustang Flats set, which are well balanced in terms of tension and tone.
The pickup in the Vintera is designed like the originals. Each pickup half has two long, thin poles. They are very 'beamy'. If the string intersects the top of the pole then the tone is quite strong and rich, but as soon as the string is off-axis the volume and quality of the tone drops away. The poles in the pickups are spaced out further than the string spacing. The G on my bass runs directly over the corresponding pole, whereas the D in particular is off-axis and weak sounding. Adjusting the pickup closer only half solves the issue.
I will take the covers off the pickup coils and stick it back in my bass to try and demonstrate what is going on.
The Nordstrand pickups put a lot more pole piece under each string, and the coils are slightly bigger so the tone should be a bit more thick and full.