A loose connection can be madenning

 Put the baffle back on.  Measured.  Wait what happened to my dual humps?  The peaks and the valley between them are not nearly pronounced enough.  Oh right need to actually tighten the baffle first. :)

There they are!

Why am I taking so many measurements? Because it is easy and it ensures I know when something gets disconnected. Don't want to reassemble the speaker only to find out that I've already broken something else.

Can I just say, yet again, how lucky we hobbyists are to be living in a century where speaker impedance measurement tools are affordable and plug into our USB ports??

Back in the 50’s you would have had to use a hand adjusted frequency source, an amplifier, a scope and resistor and to the math yourself. You didn’t even have a spreadsheet to help you. Then we got to automatic paper plotters which were far too expensive for the average lay person to have lying around. And if you were using a tube amplifier you had to keep an eye on the output voltage too... it is no wonder that companies like Infinity put out speakers with terrible impedance signatures. Optimizing for frequency response and impedance all at once with so many drivers was a real nightmare. It is so much easier for any speaker designer today.  Also, there is absolutely no excuse for designers to do this today.

I also want to say, if you are a hobbyist who wants to do something inside a speaker you 100% need to invest in an impedance analyzer. They are cheaper than a lot of capacitors and coils you are going to try to spend money on, and well worth it.

+1 @erik_squires