Congratulations on a fantastic bass. I just love Wals.
Re the controls there is one volume and one pickup blend knob which you have found.
The other two are parametric type tone controls one for each pickup so you can create the tone you like from each pickup then use the pickup blend knob to mix the two together or use just the bridge or neck pickup. As you say there is an active/attack push pull function on the two tone knobs which again adds to the variations available.
Once the controls are mastered which can take a little while, you will be rewarded with a vast array of tones.
So lock yourself away with your new toy and go experiment.
If I have not explained the controls quite right then I am sure someone else will clarify
Regards
Matthew