3-4 dB dip at crossover region: what should I listen for to hear it?

You don't want to hear it, because then you can't un-hear it and you'll be looking for new speakers.  If you like the sound, close all ASR webpages, and just listen to the music.

Believe it or not I actually would like to hear it, if only so I can better relate measurements to performance. The thing is, I’m not sure these speakers are right for me (too soon to tell - I always take a long time to adjust to new gear), so while I’ve got them I might as well learn from them. These are my first horn speakers so I bought them as much for the experience as anything else.

Why do you want to listen for something that many consider a flaw? Just enjoy the music and if you can't because of some perceived problem, buy some different speakers and do not look at the frequency response. Sheesh.

Not sure if this is right or relevant (better informed posters like Mr. Squires, please correct me if I'm wrong), but such "dips" (i.e., "3-4 dB at about 1.5 kHz") are not analogous to, say, missing a shade of blue between Cerulean and Cobalt. What I mean is that the missing shade of blue will jump out as a gap to the eye. The "missing" decibels "around" 1.5 kHz, however, are as it were blurred together with frequencies just below and just above. It's not a discreet gap. I learned this by using a crossover to send "only" frequencies below 50 Hz to my subwoofer. I found that if I shut off my main speakers, I still got sounds out of the sub that were clearly above 50 Hz. A knowledgeable friend described this slope phenomenon to explain why. If this is a poor explanation, I would appreciate gettin' schooled by someone better informed.

BTW, the SPL meter linked above (by elliot...) is only A-weighted. But C-weighting is much more relevant for music, as it doesn't discriminate against low frequencies. If you are listening to music loud (say, 90 dB or more C-weighted), and switch your meter to A-weighting, the level will drop significantly (to 80 dB or even less). 

One more thing. Several here have objected to measurements in principle, appealing to what your ears like as the only relevant standard. Well, yeah, of course. But that doesn't mean measurements are irrelevant. I don't want to open up this always contentious can of worms, but the fact is that measurements are used by the scientific engineers who design the equipment, and they do correspond, in "objective" ways, to our "subjective" ear-experiences. Duh. If you simply reject the relevance of measurements altogether, then you reduce the audio experience to one of taste alone. If you do that, this forum becomes nothing more than a place to share your enthusiasms. 

Speaking for myself, I appreciate posters like Erik Squires because they provide more than mere opinions, more than mere personal preferences. There are correlations between measurements and subjective listening which, at least in principle, bypass the pitfalls of mere judgments of taste.