According to Luxury and Precision, the variable LO of P6 Pro is implemented as follow:
Why is the LO volume of P6 and P6PRO adjustable?
The definition of LO is the analog audio signal directly output by the DAC after LPF (low pass filtering), without going through the headphone amplifier. The LO of most players cannot adjust the volume, such as the LO of L5, LP5, LP6 series, because the analog volume circuit is between the LPF and the amp circuit.
But then we found that the input level of many car audio is not standard. Once the input exceeds 1.5V, it will "clip the top", which will have a greater impact on the sense of hearing. Therefore, on the P6 and P6PRO, we deliberately put the analog volume circuit after the LPF. Before PO, the LO is connected behind the volume control circuit, which is convenient for the user to lower the level, so that the ideal distortion will be achieved when connected to the car audio.
With the above, would
@Andykong please advise whether it can be regarded as real pre-out?
I have just tried both Line In and Preamp In of C9 with LO pf P6 Pro for a couple of songs, under tube mode and class AB with Unique Melody MEST and Satin Audio Zeus cable. Both modes have preserved the neutral, analog and effortless presentation of P6 Pro. For Line In mode, the background is darker and cleaner while for Pre In mode, the weight of notes is stronger with more reverb and stronger bass. Comparatively I enjoy the Pre In mode more upon more cautious audition but the volume control at C9 under Line In mode is more precise.
There are too many options of C9 for me to explore (Line / Pre In, balanced / SE, tube or solid state, class A or AB) with a number of DAPs that I own as sources (P6 Pro, LPGT Ti, DX300, Cayin N3 Pro and Sony 1A). As commented by
@Whitigir, the C9 worths every penny and I found myself difficult to come back to standalone DAP setup without C9 when listening my music at home.