I've been very interested in these preamps, and I'm a little curious why you say that. I've owned several Wal basses with filter-based preamps, and I always sort of thought of them as a "supercharged" version of a simple passive treble roll-off tone control. What do you get from the tone control that you don't get from the filter?
That is indeed the tough thing with a filter-based preamp, and one of the reasons I always appreciated the Wal's well-marked knobs!
You really might look at Alan's original 2-filter version. Having the ability to have separate filter settings on each pickup made a huge difference on the Wal.
Mike