I have an SVS PB12 Plus which I have been very happy with. Every now and then I like to check my speaker levels to make sure everything it should be. I use the Sound & Vision callibration disc and an spl meter to do this. For some reason, when trying to set my sub speaker level now, The reading on the spl meter(about 65db) is the same with the gain all the way off as it is at the half way point. Could a bad sub cable be causing this?