Since Twister6 has raised an interested topic regarding volume would have been much lower if run it off the BAL output (or maybe high gain will also cause similar effect), I'll take this opportunity to explain the volume and gain control of C9 in detail. This is one of the critical designs that facilitate C9 to work well with both IEMs and headphones and the implementation is way more complicated than we anticipated in the very beginning.
Gain Control, Volume Control and Input Sensitivity
The C9 volume control is divided into two parts. The front-end control is a high quality 4-channels ALPS potentiometer, this will provide a firm, reliable and familiar rotation control experience to our users. The back end of the volume control system is a PAIR of low distortion 130 steps resistance ladder stereo electronic volume, MUSES72320, this is a very high quality volume control chip, it is more sophisticated that then PGA2311A used in N8 DAP. The PGA2311A has internal op-amp that can’t be bypass and that will introduce a small amount of background noise and its own tonal characteristic to the circuit. The MUSES72320 integrated with our discrete headphone amplifier circuit completely, instead of relying on internal Op-Amp. Each MUSES72320 is equivalent to 4 potentiometers in one chip: 2 for stereo volume and 2 for stereo gain, we need to install a pair of them for fully balanced implementation.
Although it comes with its own gain circuit, we want to further enhance the gain control of C9, so we have added a preliminary gain control circuit before the MUSES72320 electronic volume control, and use a MCU to select between internal gain circuit and our own gain circuit according to the volume level setting. For the record, we have two MCU in C9, one for volume/gain control, and the other in Power management circuit.
The ALPS + 2xMUSES72320 in C9 delivers a volume control system that is very precise, highly linear, and extremely flexible so that you'll have enough "room" to adjust your volume from highly sensitive IEM to full size headphones. During the critical volume range (~63dB to -5dB), we offer 0.5dB attenuation per step, and that span across from slightly before 9 o'clock position to 4 o'clock position of the ALPS volume knob, roughly speaking.
Take Alex's little experiment for example. When you change from single-ended mode to balanced mode, the output power will indeed increase significantly, 4 times more at 150+ Ohms, or more than 3 times below 150 Ohm. However the volume position will not go down linearly to 1/3 or 1/4 of original position. That's because power vs dB is a logarithmic function, not a linear function. To quantify the dB vs power relationship, 4 times more power will increase volume by 6dB (check WiKi for detail explanation), and in C9 volume control, that will be 12 steps, or from 12 o'clock (61/130) to 11:15 o'clock (49/30)
Another factor that affects the relationship between gain control and output power is input level. We have documented that in the input sensitivity section of our specification table.
If you are in Line input mode, C9 can deliver the stated output power in low gain IF your front-end can meet the output level as stated in the Input Sensitivity of our specification table, that’s why didn’t list high or low gain setting as an variable in power rating. For instance, you only need 1.5V Single-ended and 3.0V Balanced in high gain for C9 to deliver the rated output power, but if you switch to low gain, you need 3.0V Single-ended and 6V balanced for C9 to deliver the same maximum rated output power. Similarly, if you are using PRE-amp input mode and your preamp (or phone out) offers adequate output level (e.g., 8.4V from A02), you can achieve maximum output power at low gain.
The question is, do you need the extra power provided by high gain even when your output level does not meet the requirement as stated in the input sensitivity table? This is more a headroom issue, so you can consider high gain as extra 6dB headroom at difficult load rather than how many watts is needed to produce 100dB loudness at 1 kHz. On the other hand, headroom is a personal requirement because it also depends on music genre, dynamic range of music passage and/or person preference in music presentation. In my person experience, if the sensitivity of my headphone is not too low (e.g, enough sound pressure before 3 o'clock position), then I should have enough headroom even at low gain. For your reference, from 3 o'clock position (step 109/130) to max volume is roughly 13dB. I prefer to stay in low gain all the way because low gain offers slightly better dynamic range and SNR over high gain (again, check the specification table for the exact different at different mode).
!!!! Quick!!! Join me!!! Head first 
