Why are my Presonus E4.5 monitors producing such low sound?

Hello. I have a pair of Presonus E4.5 monitors/speakers. They worked fine when I bought them in 2014/2015. I used them primarily to play play music recorded on Studio One and for basic Internet use. But after a year or so of solid use, I noticed that the volume was considerably lower. It wasn't an issue on my PC, such as the mixer's volume being down. Here recently, I moved. I just set up the speakers, and the volume is even lower. It is coming out of both speakers, and it is equally low. The speakers are set up properly. The metal ends of the red and black cables are attached properly. The guitar-type cables that run between the L and R outputs on the back of the left monitor are plugged into the Audiobox USB interface correctly. I'm beginning to think these things are just defective. This is the last time I'll see if I can find an solution for this problem before inquiring about a warranty. Thanks. Recent Questions...