Here is a thread on the armor layering topic, where David Pulver himself actually answers questions (see post 24)
http://forums.sjgames.com/showthread...=115818&page=3
Here is a thread that may have been what I was remembering
http://forums.sjgames.com/showthread.php?t=77538&page=2
See post 11
So my remembering (based on that thread) is that your armor already comes complete with the appropriate underlayers, so any extra added ones do eat DX penalties