I've got a fairly loud hum coming out of my Gibson GSS50. I flick the amp on and it kinda pops then it hums.  The hum doesn't change no matter how loud or soft I turn the volume.  It's also in all the inputs both channel 1 and 2 (there are 2 inputs for each).  What could be causing this?  Could it be a grounding issue?  How do you ground an amp better.