If I measure with one of the main speakers, there shouldn't be an amplitude dip past the cross-over point, right? In other words, my FR should be relatively flat at the same amplitude all the way up to 200 Hz, right
That’s theoretically true on paper, but when the sound is subjected to the room’s anomalies it may or not be readily apparent. The best way to check it, if you’re really worried about it, would be to measure the sub and main speaker separately, then together. At the crossover frequency, the SPL level should be higher with both together than separately.
And why measure only one of the main speakers with the sub and not both simultaneously?
Because if you don’t get the mic perfectly centered between the two speakers, there is a slight delay difference between the two speakers. That shows up in the measurement as comb filtering (i.e., sharp, ragged, up and down response). You’ll probably get plenty of that anyway just from room reflections, but no reason to aggravate it.
But it ultimately depends on the purpose of the reading. If you’re checking your speakers’ and sub’s frequency response, i.e., trying to identify problem areas where response is not linear, then the above approach is best. If you’re wanting to check the relative level of the mains compared to the sub, as in adjusting the blend between the mains and subs, then run both mains together.
Regards,
Wayne