Active preamps are designed to have a certain frequency range for each control, as well as a max boost and cut for each control. Sounds like you are just not happy with the max cut for mids and treble. This isn't really because that preamp is "cheap" or lower quality, it's just a design decision that's made at the time the electronics are designed. It might seem stupid to limit the min/max range of the control - more is better, right? But there's a tradeoff - controls with very large boost/cut ranges tend to be very very sensitive (a small tweak of the pot makes a huge difference in the tone) which can be incredibly frustrating.
That said, it's pretty trivial to add a passive tone control, either on the output of the preamp or on the input (between the pickups and the preamp). Just a normal old pot with a normal old cap. If you put it on the input side, you can use typical tone control values (250k or 500k pot, .047-ish cap). If you put it on the output, you need to scale up - 25k pot and .1uF cap would be typical. Adding this control leaves you with an "active" bass, but gives you an extra degree of treble cut. Personally, I'm a big fan of this, though I typically leave the passive tone control in the control cavity, as a mini trim pot, and use it to set a "baseline" tone. Then I use the active tone control on the EQ to tweak from there. But you can have the new tone control on the outside of the bass like all the other pots if you want.
That change can be made independent of installing an active/passive switch. An active/passive switch can be added also but it's hard to speak to how to do so without knowing more about the preamp. If you can show us photos we can probably figure it out. (unless someone comes along who knows that preamp well enough).
