Subwoofer troubleshooting

I’ve recently started a home theater, and am using the Denon AVR-S650H receiver with Klipsch surround sound speakers and Accoustech subwoofer (PL-200II). Everything sounds great, except for the subwoofer. It sounds very weak, and I can barely get any sound out of it despite ensuring proper connections, using a new subwoofer cable, switching out receivers, and ensuring that the subwoofer actually works by taking it apart and testing it with a 9V battery. After the very first setup, the subwoofer was working very well, but after about a week, it randomly stopped working. I then took the above steps, and still, its very weak. Does anyone have any advice short of hiring a professional AV company to come out and try to troubleshoot (if they are even willing)? Thanks!