Just a minor correction, its AKM AK5554VN, and as the final number implies, it provide the 4-channel ADC to the Motu M4. The Motu M2 uses the 2-channel AKM AK5552VN instead.
What I don't understand, is that this is a high quality ADC capable of delivering 768 KHz @ 32bits and clean frequency vs THD, and yet it have some weird stuff happening on Amir's review. ¯\_(ツ)_/¯
Analogue inputs from the front panel are buffered by couple of OPA1678 op amps and the gain is adjusted by the dedicated THS4522 and THAT6263 chips. I am guessing that the first one acts as an input buffer and is having a fixed gain, while the second is changing the gain in 3dB, per manufacturer datasheet, although in real-life M4 mic inputs are fine adjusted by 1dB and not by 3dB. Perhaps someone else could jump in here with some thoughts about how the above two chips might be used inside the M2 & M4.
Well, both these preamps are stereo, and the M4 have 4 inputs to power, which can mean:
1) The inputs on the back share the preamps with the front inputs
2) The inputs on the back are not amplified, on the inputs on the front
3) The THS4522 operates on the back inputs, THAT6263 operates on frontal inputs.
Looking at the PCB, the XLR combos have identical circuitry, with the THAT6263 between them.
Also, the Headphone Monitor knob is a digital step controller that act on 1db steps. IIRC, the Input Monitors act in 0.05db steps instead.
EDIT: A video of said step controller on the unit, plus an additional pot on the signal chain.