I own the 400M monoblocks...
I have listened to the Moon's 240i, 340i and 600i integrated and the 330A and 400M amps.
I specifically selected the monoblocks over the 700i because I listen to music louder than most.
The guidance I was provided by my dealer and reinforced by the Moon rep was 700i provides a bit more detail and finese; while the 400M has the power and punch. (i.e. 400M has a mullet and the 700i is wearing a suit).
Moon has a trade-up program; so if you buy the M400s they will accept them in for 100% of the retail price within 1 year and 75% within 2 years to trade-up. To me buying the 400M was a risk free proposition
I do often listen at 55-60 on the volume dial (out of 80) I am pleased with the detail and dynamics - it was significant upgrade over my McIntosh MC-302