Thanks for the input! I know abstract algebra isn't too relevant to statistics, but I want to get a good subject test score. Plus, so far I'm enjoying it--not a bad reason to learn about something, right?
I should also point out that I've taken several stats courses, including mathematical statistics and a master's-level probability course. So I think I'm okay on the statistics front. It's mainly math that I want to strengthen.
Multivariable calculus, linear algebra, and numerical analysis all sound like good ideas. What I'd really like to know is if there are any exams out there that I could use to certify my knowledge of those topics without having to pay for and take an entire course. Just to give an example of the sort of thing I'm getting at: The Royal Statistical Society sponsors a series of statistics exams, and passing all of them has some recognition in the UK as equivalent to a bachelor's degree in statistics. I'm hoping there might be something similar for math. I'm sure these hypothetical math exams wouldn't be as helpful to my resume as A's in actual courses, but they might be good enough for my purposes. I just want to save time and money if I can.