I think you should try to reverse one of the element to see if it solve your problem.
for the pressure, a bass player from talkbass gave us a good solution: insert a saxophonist reed between the pickup and the bridge. By this way you can control the pressure you give to the pickup and check the best sound you can get.
I bought that pick up a few years ago to a shop called Alienor
http://www.alienorlutherie.com/
and can remember I called the company who is located in Paris and get a few information from the boss (it's a small company)
like the trimpot, or the pressure to give to the pickup, the best was to go and see him in his shop, but I didn't.
I don't succeed to find now the address or something.
I think you should probably get the new address by calling few French shop
or ask to our "national French Talkbass" that you probably know:
http://www.contrebasse.org/index.htm
Here is a link to a good musical site too
http://fwf-system.audiofanzine.com/
I can't say if FWF is better than an underwood, I just tested the two on a different bass, place, amp, and strings.
The underwood seems to me dryer, but once again it was a different bass, etc. ……
Bonne chances dans tes recherches.
Alain