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.