"Ignore the app. Make sure the amp’s positive (red) terminal is connected to the speaker’s positive terminal and the amp’s negative (black) terminal to the speaker’s negative terminal. On both speakers. If one speaker was connected backwards, the bass is out of phase and will cancel out."
Maybe not. Maybe the app is correct and one of the speakers has reversed magnetism? This does happen and I have a professional JBL woofer like this which is reported by one of their authorized service reps as happening for about 1 in 50 woofers!
I just couldn’t get the two woofers to phase properly when connected the same. I discovered that when a compass is held up to the front of the two woofers, one shows north and the other shows south! YEAH THEY DO!!!
Or maybe the wires are crossed inside the cabinet and the external connection would appear correct?
A 1.5 volt battery is probably the best way to check.