I had an Anthem Sl installed in late 2010. In the past few weeks I'm getting a lot of low freq hum. If I play with the cable at the preamp plug I can kill the hum but I have no low end response, like only the mic is working. I've tried different cables and have reset the saddle. I'm assuming it's a contact problem at the preamp. Any ideas what I can do to fix this? I'm out of warranty.