I'm wanting to add the Noble Dead variant from Classic Horrors Revisited to a vampire. The vampire already has +4 Channel Resistance and the Noble Dead variant grants +6 Channel Resistance. Do I end up with a vampire with +6 Channel Resistance or +10 Channel Resistance?
I tried Googling this question and this thread was my #1 hit on my Google search results. Google is fast like that.
I'm guessing they probably don't stack and I should just take the +6, but does it actually say that somewhere?
Take a look at other monster variants: PRD Variant Monster Index
The description always says "gains ability XY" or "replaces ability XY with YZ". This is the best RAW i can find.
In my opinion both are some kind of "template" or "creature/racial" bonus. So they dont stack and you end up with a +6 in your example.
In 3.5 Channeling (Turn Resistance) effects stacked because they were an un-named bonus. Unless there is a rule explicitly stating they do not stack, they would as that would have been grand-fathered in to Pathfinder.