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

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.

mlsstl’s comment dovetails with something I wanted to add. I use unamplified acoustic music as my "personal reference," too. I do like rock, even loud rock (e.g., Tool), but mostly listen to so-called "classical," and I play cello and acoustic guitar; my wife plays piano and my daughter violin. We hear live acoustic instruments in my audio listening space every day. We also sing, my daughter professionally.

Still, here’s a lesson of some kind, I think. I’ve got five pairs of high-end speakers, and two systems (one mostly for movies, in the library, and the main rig for music). Every now and then, I set up one—or even two—of the "extra" speaker pairs in my main listening room in such a way that I can fairly easily switch between them and my favored pair. And let me mention that my favored pair (Scientific Fidelity "Teslas" made in the late 1990s) are rare probably because Corey Greenberg in Stereophile killed the company with a very negative review when they first came out. One of the other pair are highly regarded Von Schweikerts, and another pair won all kinds of awards from Stereophile and other respected places, measuring flatter and with less distortion than any speaker at any price ever measured to that point in the anechoic chamber of Canada’s National Research Council (that’s a hint about its identity). However, of all my speaker pairs, it’s that last one I like the least. And remember: my ears are trained to prefer natural acoustic instruments in the very same acoustic space as my audio system utilizes.

So what’s the "lesson" here? Maybe that listening to recorded music is just a different experience from listening to "the real thing" (if that means: live acoustic instruments, voices, etc.). To the extent that this is true, measurements may actually be misleading—as they are for that speaker I just mentioned but didn’t name.

That cable comment is popular with me!
Also the remarks about room acoustics. I was going to write something of the kind myself. Room acoustics are at least as important as the speaker technology for the final sound.