True - only issue is that as @Minigun explained in Windows M4 shows as two separate audio devices - one for 1-2 output (volume controlled) and another for 3-4 output (line out at max power), while on MacOS Motu M4 shows up as a single 4-output device where only channels 1-2 play system audio. With some additional virtual device software it should be possible to route system audio to both 1-2 and 3-4 outputs. Probably in Audio MIDI Settings / Configure speakers it would be possible to set Left and Right to be channel 3 and 4 but this won't be very convenient.
I'm choosing to stick with M2 for the moment, it works very well for balanced monitors and unbalanced headphone amp. As long as you remember to turn down the volume down when using studio monitors.
Fair enough. I personally hate the faff of having to remember to turn down the volume when unplugging headphones when it comes to *any* speaker amp that just doesn't do this functionality, and it always bothers me!
A pity that MacOS doesn't work properly via that route, though.