One speaker putting out -3db less?

One of my speakers is playing around -3db less than the other. I switched the speaker cables to make sure it wasn't a channel in my amp. This just recently started. I also turned my sub off when I checked it. I used a spl meter to do so. My amp is a B&K reference 200.2 and my speakers are Paradigm Studio 40 v4. Any ideas what can be causing this? My speaks are only 2 years old and I do occasionally crank them, but never all the up.

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.
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.