REW & Measurment mic, confirmation

Both usually, but I like to start without filter in place at low volume, making sure you do not put too much low frequency thru a comp driver, for example.

The idea being the natural response of the driver without crossover filters will sometimes assist in picking a crossover point.

For instance, in a standard 2-way 12" woofer + 1" comp, I'd want to know the natural acoustic rolloff that the woofer has in the top end, and the comp/horn combo has in the low end, prior to adding filters. Picking then, ideally, a point where both have low distortion, equal on/off axis response that would only be improved by electrical crossover filters.

Sometimes seeing the natural driver rolloff of 6db or 10-12db per octave, for example, will determine the electrical filter one could use, i.e to get a 0db sum at crossover, with 24db per octave acoustic slopes, would require only a 12db electrical filter, which might combine with the natural driver rolloff, to form the 24db acoustic crossover slope if that's the intended target.

Intended application may also play into the above, for example using the same pair of 12" + 1" combo for HiFi listening levels vs PA listening levels, one might be able to crossover lower in the HiFi setting with lower levels, thus gaining closer centre to centre spacing between the drive units.