One speaker putting out -3db less?

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.

Move the speakers to the center of the room when you to your test. Are your speakers bi-wired? You can also check upper and lower frequencies left to right to narrow down a problem.

Are the speaker cables the same lengths?

These are all great points. I used a handheld spl meter in the center and moved my balance left and right.It's a noticeable difference. I noticed my soundstage had moved slightly to the left, particularly the vocals, that's what caused me to check it. I'll do a orw thorough check later tonight. Also, don't know if this could have done something, but I've suddenly had a terrible static issue. I grabbed my speaker a couple of tims now to adjust it and been shocked to the point it sounded like my cd skipped.

You need to use white noise to test and mono if playing music.

Look for a tweeter that is not working.

Make sure the connectors are all clean and tight.

Use contact cleaner and clean the volume and balance controls. Shut off power and twist knobs multiple times.