Interesting article, although the LC circuit (resonant low pass circuit) is something I would teach my students as a phys lab TA, I feel like the key is the inductor and the cap more than the pot. The issue with the MRB costing more was that the switch in the MRB was to switch between 3 different cap values: http://www.voxshowroom.com/ct/amp/us/features/mrb.jpg which would be expensive. The wah circuit changes the amperage (current) to the cap instead, making it act like a different value. (explained here: http://www.geofex.com/article_folders/wahpedl/wahped.htm#wha... )

Also a side note, if you want a variable wah without the footpedal you have a couple of options: http://diy.smallbearelec.com/Projects/WMouse/WMouse.html or build an envelope follower (which is neat because it triggers off of the amplitude of your signal, aka how hard you are strumming, which is nice for people w/ bad foot coordination) with a settable Q/freq: http://buildyourownclone.com/products/envelope-filter-fixed-...

Other neat innovations: zvex wah-probe (theremin circuit controlling the wah freq) and the zvex seek wah (a sequence of different wah settings you control). anyway, wah-wah's are fun and chances are if you did a set of physics electricity labs you built one already, just without the transistor choking the cap.