My bass has Europa electronics, I'll share my experience with it.
As StephenR said, the Q switch introduces a little bit of resonance just below the frequency where the filter cuts off. This sort of frequency response (flat response at low frequency, a peak, then highs rolling off) is a basic characteristic inherent in conventional pickups. Alembic pickups are different - they have a very wide-band flat response which provides the raw material for the filter to work with. Thus the filter with Q on can be configured to reproduce the characteristic sound of a variety of pickups just by adjusting the filter frequency.
In the Europa electronics, the pickup pan also affects the sound. Blending different amounts of the two pickups produces some fairly dramatic changes in the tone the filter operates on. There are also what I call the "tone preset" switches (the bass and treble cut/flat/boost switches) that affect the overall tone.
I have a few different starting points I like on my bass. One is to turn the pan all the way to the neck pickup, then back just a bit to blend in a
little bit of bridge pickup - there's a point at which I hear it get just a little deeper and more powerful. Then I turn the filter down very roughly 1/3 of the way from fully open to where it "takes the edge off" the bright Alembic tone. Depending on exactly what I want, sometimes I leave the Q off and the tone switches both flat; or I turn the Q on and open the filter just a little but switch the treble to cut. This general range is good for songs that want a vibe similar to a Fender Precision.
Another is to set the blend in the middle (as it happens, the detent on this knob on my bass is in just the right place, it can be fine tuned with the internal trimpots on the preamp). This gives a tone that I think of as "chunky" or "juicy" (I know, those words seem contradictory, please excuse my mild synaesthesia). With the Q on, the filter sweeps through a range of Fender Jazz or Rickenbacker bass tones.
If I'm going for a darker tone, I roll of the filter more. Beyond a certain point, I much prefer to turn off the Q switch, it gets too "honky" (for lack of a better term) for my tastes.
And of course sometimes I just want that really hifi Alembic sound, and I run the filter (almost) wide open, Q on or off to suit my mood.
I've found all these tones just by fiddling with the controls and listening; I'm still (after almost two years with this bass) finding more nuances. I'll suggest doing whatever you like to do to put yourself in a state of mind for really appreciating the sound of your bass
and spending some time exploring. Don't be shy about flipping the switches and turning the knobs! If you think you like your Europa now - well, you're going to like it more.
