Duplicating objects and then modifying them will allow me to create new masters off old objects but I've found this isn't true for master shaders.

I cannot build a new shader off of an old one.  When I select to make a new master shader, no new shader is created.

I've also noticed that I cannot change this option in the preferences dialogue.  I can select the check box for the "same choice for all masters" check box but it makes no difference which option I select for master object ambiguity.

This isn't fun.

-Kix