Just purchased my first Sub, its one of the Dayton 10" 100w versions. Sounds great but it seems to have one annoying problem. If I get close to it I can hear a hum, even when its not turned itself on. By that I mean its set to auto-on but the light is red and the amp has not powered on yet. Parts Express sent me another amp and I changed it out, it seems maybe a bit better but the hum is still there. Is this normal? Would appreciate any advice.