These are the response curves for the Tone TwEQ. You will note that the mid frequencies are centered at 1K. The Bass and Treble filters are shelving types. So the Bass filter easily covers all of the lower frequencies you will use with a 5 string bass. However, if you change the Mid filter by changing the cap and resistor values, you will cause that filter to overlap the bass filter even more than it currently does.
Now, to discuss the use of this as an “ON BOARD” preamp. Keep in mind that this device uses four pots. Each filter has a pot for boost and cut and there is a final volume pot which sets the output level. The output can be much higher than unity and it is also a low impedance output. So it can serve as a preamp. Because it has 4 pots, it is probably not a good choice to mount inside of your bass but would be great as a pedal!
I have had several requests for building a combo floor pedal that incorporates the Tone TwEQ as a final EQ/preamp/low impedance output with multiple outputs serving as a DI box with balanced XLR and TRS outputs as well as Normal & inverted outputs with phase switch, etc. Also, keep in mind that many bass players use compressors and even distortion/fuzz. These can also be combined into combo pedals that our builders create.
I hope this answers your query and also gives you some ideas for future builds.