How can I tell if my speakers are wired "in phase"?
All speakers have “polarity”, that is a speaker wired to the industry standard will move forward with a positive DC voltage applied to the positive terminal of the speaker and a negative voltage applied to the negative terminal of the speaker. While there are standards, there are also deviations from standards, either by legacy (ie. early JBL drivers), by faulty repair (incorrectly wired cabinet or defective recone parts) or a manufacturer choosing to ignore the standard.
If a multi-driver speaker cabinet or a pair of speaker cabinets does not appear to have the expected output or low end, it’s always a good idea to double-check that all drivers move forward with positive DC voltage applied to the + terminal (this will be the 1+ terminal on a SpeakOn™ connector, or the tip on a 1/4” connector).
If you find on a multi-speaker cabinet that one speaker moves out while the other does not move, it’s likely that the non-moving driver has failed or has become disconnected. If one driver moves out while the other driver moves in, it’s likely that the driver moving in is wired incorrectly or in a sealed cabinet is failed or disconnected and is merely being moved in the opposite direction by the air mass inside the cabinet itself. Being aware of these possibilities can often help when something is not quite right.