This very nice series appears for DSI Tempest in 2012: https://stimresp.wordpress.com/2012/02/19/tempest-recipes-first-things-first/
If you follow that thread you'll find a bunch of first principles and ur-patches for perc synthesis. It is one I should revisit too. A key takeaway: you can make perc out of lots of things.
Found this old thread after Hexinverter emailed their list to announce upcoming closure. In sadness and greed I went and bought a bunch of Mutant drums.