OK, I've spent a bit of time searching here and on wiki, etc. but I can't find anything to sum up the whole concept of impedance in audio gear. I've come to a conclusion that the impedance of the output must be higher or equal to that of the input it's being wired to, or you get buzzing and noise etc. eg My headphone jack is 300 ohm and I have no problems with 64 ohm HPs but putting this into a Line In at say, 10Kohm causes noise. BUT I have no faith in that statement unless someone confirms or denies this. Basically, what's the rule of impedance? Input = output, input < output, input > output or what?