Just some ideas …
have you checked this long discussion already? Analog Four Bass Tips
On my AK I have the impression, if I use the second filter as resonant highpass that I have to take care of my speakers.
The loudness of a voice also depends on its envelopes. Do you have a more soft bass in mind or a punchy sound? Steep and fast envelopes - particularly on the filters - can make a big change.
Additionally, if the filters are fighting against each other, because their modulation is not synchronized to support each other, the sound also could become weaker.