I lack the proper terminology to describe how they work, but perhaps the answer lies in the Bass and Treble controls themselves. For Epic/Orion (or EMW equipped basses) these controls are asymmetrical boost/cut functions, +6dB and -15dB, centered around 100Hz and 5KHz for the bass and treble respectively. That info is from the Epic 'Controls' section, so I'm just assuming it applies to both. I'm only guessing the filter doesn't filter these frequencies out completely.
*edit, found this elsewhere-
On Alembic instruments, the low-pass filter frequency control has a range of 350 Hz to 6 kHz. That means if you leave it wide open (6 kHz), you'll basically allow all useful frequencies (for a bass) to pass, and it won't really act as a filter at all. On the other hand, if you set it to the minimum of 350 Hz, then you are starting to filter out some of the highest notes.
From firsthand experience, on my fretless - it is not only possible, but in fact a very pleasant sound to roll the filter back closed to about 20-30% open, bias the pickup balance to favor the bridge, and boost the bass a bit while cutting the treble just a smidge. Or the opposite; pan more to the neck pickup, open the filter, brighten up with the treble pot and roll off a bit of fluff with the bass EQ. Or go straight up the middle with the blend, and leave the EQ flat. (there's a center detent in mine...) and dial the filter wherever you need it to be. Don't get me wrong, I love all my Alembics, and their various electronic workings, and there isn't anything on Earth that sounds like a Series bass, but I definitely made the right call on this one. It's like a modern, more adjustable take on the old Distillate circuit to me. Simple, but very adjustable. Perfect for what I do.
And that purple Elan sounds marvelous, Jazzy.
