How can the IAU 2000A vs IAU 2000B nutation comparison be reproduced?
I suspect that the problem is in your assumption that changes in $\Delta \epsilon$ or $\Delta \psi$ lead to coordinate changes of the same size. In fact, the position of the equator and celestial pole are complex functions of those angles, notably with both positive and negative terms.  If you look at Equations (8) and (9) (for the coordinates $X$ and $Y$ of the pole) in the 2008 A&amp;A paper you linked to, you can see this dependence.  For example, in the $X$ equation, there is a $\Delta \psi \sin \epsilon_0$ term, but later there is also a $- (\psi^2_A / 2) \Delta \psi \sin \epsilon_0)$ term that could at least partially cancel it for a given value of $\Delta \psi$.  There are also cross terms between $\Delta \psi$ and $\Delta \epsilon$. 

While those equations are only for an approximation to IAU 2000B, looking at Capitaine &amp; Wallace 2006 for the exact equation (Eq. 36) shows the same behavior, i.e. that there are both positive and negative terms, as well as cross terms, involving those quantities. 

So to do a comparison of accuracy, I think you would need to calculate the $X, Y$ coordinates of the pole and the position $s$ of the origin with both models, and compare those.