Familiar Drop Multiplier

Now, I know that familiars give a certain rate of drop increase, but how do we find out whether if its 1.6x or more?

I mean, it's common knowledge that Big Spider or Mutant Pig gives a 1.6x increase, and the Wolf Underling gives a 2x increase.

Is there any other familiar that gives 2x increase? or does anyone know if Castellan is 1.6x or 2x?

Thanks in advance