Although I'm uncertain about the type of Bartolinis used in the J2, I do have experience with the single humbucker Rapture 5. The stock pickup in mine was a 66BD5C-T, which is Bartolini's dark humbucker with a custom coil width. Although I liked the overall feel and sound of the instrument, I too found it a bit lacking in upper midrange presence. Bartolini's Classic Bass pickups have a more open sound, though they still might be less aggressive than some might prefer.
Here's a thread which discusses the swap to a Q-tuner BL-5:
Q-Tuner Review with Bartolini Comparison Samples
The Q-tuner definitely brings out a lot more of the upper harmonic overtones, and really opened up the sound of the instrument, IMO. As for the NTBT preamp, I'd say that it does color the tone somewhat, though the pickups are the greater overall factor. To my ear, the NTBT is fairly neutral, perhaps slightly warm, loses a little bit of air up top, and sounds a tad compressed. My bass has an active/passive switch, but I generally use the preamp for live playing. In short, I'd suggest trying different pickups before changing the preamp.
