That Digizoid tool is awesome, thanks. I ran the calculation again at 600 ohms, 92dB/mW (the sensitivity of the vintage K240 Monitor is reported to be between 92 and 95 dB/mW) and the results are still good enough, as I tend to listen to music at a very moderate volume.Everything is related to basic math here: AKG K240 has a sensitivity of 104 dB SPL/V @ 1 kHz and according to Digizoid we have this:
Listening Loudness Voltage Needed Current Needed Power Needed Safe 85 dB SPL 0.11 Vrms 0.18 mA 0.02 mW Moderate 100 dB SPL 0.63 Vrms 1.05 mA 0.66 mW Fairly Loud 110 dB SPL 2 Vrms 3.33 mA 6.67 mW Very Loud 115 dB SPL 3.55 Vrms 5.92 mA 21 mW Painful 120 dB SPL 6.31 Vrms 10.52 mA 66.36 mW
Given that O2 has a max. output power of about 7.3 V RMS, I see no problems driving K240.
My DT880 has a lower sensitivity and O2 will not drive them to full power, but I can still get acceptable dB levels to my ears.
Probably folks thinking that a simple resistors added to speakers output will make 600-Ohms headphones sound "better". I would ignore them and use a dedicated headamp with a low output impedance (even if the 1/8-rule will not matter to our 600-Ohms headset, you never know when you'll pair a 16-Ohms headset with that amp).
I don't plan on getting low impedance headphones and I don't like IEMs, but one of the many things I like about the O2 is its low output impedance.
BTW, doesn't having a source with low output impedance also result in better performance with cans that have very non-linear impedances with respect to frequency?
For whatever reason, I have noticed quite an improvement with my MDR7506 through my O2/ODAC vs through other sources. That could be in part to the ODAC being cleaner, but I think there might be something else going on, maybe the low output impedance in the O2 making the headphone more controlled?