I've been through a lot of trial and error with hollow body U-Basses. I have a fretless that came with white strings. I tried red, brown and gray strings. The intonation changes depending and I ended up back with the white strings which have the best E. With piezo pickups volume between strings has been an issue. I then bought a 2nd U-Bass with frets, a Rumbler and put on nylon/steel flat wounds. The QA on this one wasn't so good. The 5th -7th frets were high and in order to level them I had to file off about half of the hight. Since I don't live in the US sending it back wasn't an option. The 9v Fishman pickup had more headroom than the 6v, but the string to string volume was not good. So being a luthier, I started with improvements. I've always played basses with 1.5" nuts so one thing just lead to the next:
pulled the frets and leveled finger board
reshaped the neck
installed medium frets
refinished the neck
It really played well and the neck was much more to my liking, however the string to string volume was still an issue so:
installed a Telecaster size rail pickup in a mahogany sound hole cover, and made a compensated brass saddle. Then I could wired the grounds of the under saddle piezo and magnetic pickup together. I glued a booster ceramic magnet on the 15k ohm pickup since the nylon core strings only have the steel wrap.
The string volume is good and the piezo pickup works as a tuner ;-)
