This time I measured with the speaker on its back, and the mic directly over the Radian's diaphragm about 19" away.
I took two measurements: one through the unmodified crossover, and the other directly from the amp to the speaker. The measurements were the same!
So either I've miswired the crossover, or my measurement setup is faulty. Unfortunately I don't have any other speaker available for reference, otherwise I'd measure it.
Though I suspect the measurements themselves at this point, here's the crossover for scrutiny: