I hope i won't get in trouble for doing this; i am doing a copy & paste from another forum member who had a question in 2009, to which i never saw a reply (the thread is no longer active). I would be interested in the answer myself, here is the question: "I have a question for any Woo Audio owners out there. The transformer coupled designs at WA typically are rated for a headphone impedance of 8-600 ohms. This seems extremely large to me. I know at least some of them (maybe all of them) have a high/low impedance switch, but something about the switch tells me that the switch isn't changing the tapping point of the transformer secondary (like changing between 16-8-4 ohm taps on a transformer coupled speaker amp). The only way I can see this working is that the transformer has a 4 or 8 ohm resistor in parallel with the headphone load and that most of the output power is dropped across this resistor, with the switch basically diverting the input signal through another resistor such that it effectively reduces the overall gain of the amplifier for lower impedance phones. I find this to be a little bit inelegant and certainly a waste of amplifier power and efficiency. Does anyone know how the WA amps drive such varied loads from it's output transformer?"