Brandon Sanderson says that they are essentially resevoirs of power. So I guess that would make them additive. That said, he did add the provision that it was from the top of his head.
INTERVIEW: Oct 27th, 2009
BYU: TGS Midnight Release, Stormleader Dinner - Matt Hatch (Verbatim)
The Choedan Kal, does it amplify your power? Is it a limited, for example, if I have a certain amount of power does it give me 10x what I have, or is it a certain amount of power I can access?
One of the things I’ve been doing when I answer questions is that I’ve been saying that this is my understanding and putting an asterisk at the bottom that is a—I am speaking from my understanding and not from specific knowledge from the notes, meaning yes I am probably right but these are the questions I could be wrong on...this one my understanding is that it is a reservoir of power. It is not necessarily a magnification. A very weak person with a very powerful sa’angreal is very powerful. I’m pretty sure on that one, but I will add the asterisk just in case. If you send me an email, I can go back and look to make sure. But I’m reasonably sure on that one, the reasons being things that are talked about in the notes mixed with the way angreal and sa’angreal worked previously in the books mixed with two specific things that he talked about in the notes when people using angreal and sa’angreal after they become very weak or after they are weak.
Source: http://www.theoryland.com/intvmain.php?i=454 Question 4.