So the amp would otherwise have very low output impedance but there are 200 ohm resistors between the speaker output and the headphone output?
The ~2006 edition SEX amp (2.0) had resistors between the speaker output and this is PJ's commentary on the subject:
"The IHF standard calls for a 5vRMS source with 120 ohms impedance. This is supposed to be applicable to all headphones. It will produce maximum power into a 120 ohms headphone; that power is volts squared over resistance, in this case 0.052 watts. At the extremes of 600 ohm phones, or 24 ohm phones the available power is less, 0.029 watt. So for a tube amp capable of delivering 0.05 watts (50 milliwatt or mW) an output transformer with taps for the various headphone impedances in the range of 24 to 600 ohms would be appropriate, in order to preserve an appropriate load on the tube to keep distortion low.
As the available amp power increases, transformers with lower impedance outputs will be able to supply the necessary voltage. Used in this way, the amplifier always sees a high impedance load and distortion is kept low. At the same time, the noise voltage is reduced by the lower impedance output tap. For more information on this issue, look at the Bottlehead Community page; my white paper on signal and noise levels is near the top. The SEX amp produces 2 watts, which is 4vRMS, on the 8 ohm transformer tap loaded with a speaker. With the standard 120 ohm series resistor it can produce 5 vRMS easily, thus meeting the IHF standard accurately and with a very low noise floor.
There is a 500 ohm tap available on the stock autoformer. However, it would produce over 30 volts into a set of headphones, far too much for the phones or your ears to handle.
Now admittedly the IHF standard is old, and specifically it predates the widespread use of headphones with portable devices like iPods. These devices run on low voltages, and in order to obtain sufficient loudness the headphones need to be low impedance and highly sensitive. Some of these headphones do not work at their best with the IHF standard 120 ohm source impedance, being designed for a near zero source impedance and low voltages (around 1vRMS is not uncommon). Unfortunately, few headphone manufacturers will tell you this. But if you do have such a set of phones, the ideal would be a low-impedance source of even lower voltage - in other words, for the SEX amp a tap of less than 1.0 ohms, with no series resistor. It is easy enough to modify the 120 ohm series resistor to an L-pad which will reduce the output voltage and impedance for such phones, though I don't think we have documented such a modification yet. It would merely involve changing the 120 ohm resistor for another value, and adding another resistor from the "hot" headphone signal to ground. I would suggest 6.8 ohms (5 watt) and 1.2 ohms (1 watt) as widely available and suitable starting values."
For the newer SEX amp with the current hot headphones, the resistors no longer made sense and instead we just worked on making it super quiet so it would work well without the resistors into highly sensitive headphones so that maximum power could be available for those newer power hungry headphones.
