I'll try another measurement tomorrow with a different microphone (still not calibrated, but may give a better result) and see what that shows.
Could there be a problem with the cabinet causing the rolloff? The only thing I can think of is that I didn't use gaskets around the woofer and the waveguide, but I think they're pretty well sealed even so. If they're not, would that be sufficient to explain the drop off?
I built the crossovers myself, but I quadruple checked them, so I don't think that is the problem.