One speaker putting out -3db less?

While it certainly sounds that one of your speakers is at fault, it is worth a try to unplug all of your digital equipment for a minute or so, then power back up to see if this problem resolves. A current spike may have fooled your digital equipment into thinking it is using more power in one channel vs another, and it may have attenuated the signal in trying to manage it. Because you switched speaker cables and the volume of one speaker remains attenuated, this may not help, but is worth a try.

Where are you putting the spl meter? What type of test signal are you using? A 3dB difference at the listening position could be easily caused by the room. Switch the speakers and see if you get the same results.

At what frequencies does this 3db loss appear? As Herman suggests you might just have a difference caused by boundary reinforcement unless the loss is across the bandwidth. Also, are you getting a substantial shift in the center of the soundstage to the stronger speaker? Or do you must detect it with the sound meter and in the frequencies 500hz or lower?

The best way to measure the speakers, if you are not already doing so is to put the meter about three feet in front of the center of each speaker and chart the results there as opposed to the listening position.

You can refine this further to identify which driver is failing, if it is, by measuring each driver with the mike right in front (a couple of inches) of each driver and charting the results. Be sure to use a secure stand. This is not a 'hand held' procedure.