They switch the coils on each pickup. Series/single/parallel.
With regard to the tonal range from the filters. The lowpass filters in your version of the EQ01 go as high as 6.5kHz the newer version was brought down in line with the current DFM to 3.5kHz. This increases the resolution of the pot giving more control over the mids and it also reduces the overlap with the highpass filter. While both the EQ01 and DFM share the same control layout the newer restricted version of the lowpass filter does make it easier to dial in. Less being more in this case. This was done after the EQ03 where myself and John took things a bit to far. The preamp was hugely versatile but difficult to use. We only really worked that out once we went to far. Dialled it back to make the DFM which would be the EQ04. I don’t see it being developed any further. As far as I am concerned it ticks all the boxes I set out to tick. It took 4 versions over 9 years a fair bit of cash a few mistakes and me being the blight of John existence. Full credit to John East for putting up with me and his brilliant design work to get me to where I am and wanted to be
Regarding using it.
I only use about 20% of what the preamp can do. That produces what I like so it is what I use. However the other 80% has to be there because it may well produce the 20% you like. Don’t see anyway round that. If you want a versatile bass, and that has always been my goal, then there will be a degree of complexity and redundancy in the system. Bottom line use what you like ignore the rest.
I look on it as a bass with two tone controls one for each pickup. I setup what I like on each pickup and use the blend. The blend is your friend. Because of the independent EQ it is way more powerful than in a 3 band EQ. All you have there is the difference between the pickups and their placement. With the filters the two pickups can have radically different sounds making the blend much more important. Live the blend makes changes easy. However once you have spent time with the preamp and I am the first to admit it has a learning curve changing on the fly is certainly possible I know many players who are totally comfortable with it live. Equally there are people that just don’t get them or get on with them which is why the East Uni-Pre is always available in all my builds except for the Uber Series.
The other thing to point out is that the filters give you an option you don’t have. You have at least a 3 band cut and boost EQ in your amp Filters give you sounds you can’t get with a 3 band. Conversely a 3 band is better at some things than the filters. You have the option of best of both.
And finally for those that wish to know. Flat in a filter preamp is not entirely possible because the filters are engaged all the time.So the control setting for flat are not all in the middle as would be the norm. However you can get close with the following. Both lowpass filters fully open, upper ring fully of. Upper ring on the highpass fully of. With the active/passive that is available with the DFM A/B the above with the passive and tone fully up there is not a lot of difference. So should you want to try flat that is as close as you can get.