470µf is a safe value. This means an electrolytic capacitor, unfortunately. Film capacitors in this range would be huge and expensive, whereas electrolytic are small and cheap. The compromise is to use a small valued film capacitor in parallel with a large valued electrolytic. This works up to a point; some electrolytics exhibit a rising impedance with frequency at as low as a few thousand hertz, which would demand a much bigger film capacitor; second, some electrolytics suffer from so much leakage current that the ensuing DC offset at the output could be dangerous to the headphones. The best solution is (1) to use the best electrolytic you can get and (2) bypass it with a high quality film capacitance.

One extremely fine example of a good electrolytic capacitor is the HFS type from Panasonic. This capacitor has very low series inductance and very low leakage current 62µA. The fact that it is reasonably priced and readily available from Digi-Key does not hurt either. The bypass capacitor should be at least 1µf  in value. (The reactance of a 1µf capacitor would equal 32 ohms at 5,000 Hz.) The down side to this capacitor choice (Panasonic HFS) is that the capacitor is available in only a 63 volt rating. In tube terms, 63 volts is nothing. A 200 volt capacitor would be the safest bet, but the leakage current would run on the order of 1-2 mA.

Because the gain stage is DC coupled to the Cathode Follower, the plate voltage of the first stage should center at 50 volts to maintain a low enough voltage at the Cathode Follower's cathode for safe use of the HFS capacitor. The diode that connects the Cathode Follower's grid to its cathode is essential, as it protects the output capacitor from over voltage at turn-on, which leads us to the next topic.

Why a 6922?
The tube chosen was a 6DJ8 type dual triode, as it boasts both a relatively high gain and high transconductance and it works well with only 50 volts across it. (Other tubes could have been used in building this line stage. For 9 pin fans, the choices are few: 12BH7, 5687, and 7119. For octal fans, the best choice is the 6BX7.) Other factors in choosing this tube are its ready availability, sometimes low noise (many are microphonic), and low heater current demands.     

How much plate voltage and current to use?
Because this circuit makes tight demands on the operating voltage points, less latitude is available. In order to produce a 1 volt peak swing into a 32 ohm load requires 30 mA's of current: 1 ohm / 32 ohms. This value sets the output stage's idle current. At full output, the Cathode Follower will swing from 60 ma's down to 0 mA's of current. The load will see ± 30mA of current, which equals 14 mw of output. For the first stage, a 50 plate voltage and 5 mA's of current will produce 0.5 watts of dissipation for the triode. For the Cathode Follower stage, a 100 plate voltage and 10 mA's of current per triode will produce 1 watt of dissipation per triode.

Which sort of power supply should be used?
As this circuit has been optimized for simplicity, the power supply should be equally simple. This circuit works in a Class A, single-ended mode of operation, which means that the idle current is high and varies little. One trick would be to make a power supply from a flat-pack transformer. These transformers are available with a 48 VA capability and offer wonderful high frequency isolation from the wall voltage noise, as the windings are not layered upon each other as are those in a conventional bell case transformer. Such a transformer usually comes with four windings: two primary and two secondary. The primaries are there to allow use at 117 VAC, when wired in parallel; and  use at 230 VAC, when wired in series. If one of the primaries were hooked up to a bridge rectifier instead of the wall voltage, it would yield, under load conditions, about 155 VDC. The secondaries can be used to power the heater string.