I own 400Ms - they run significantly cooler than my previous McIntosh MC 302 and NAD C375BEE. They can get warm to the touch versus hot.
My space is bigger and I commonly listen 75-85 dbs. I have the Moon 390 and Rega P8 and have SF Olympia Nova Vs.
I can’t compare the difference in sound between the 400M and 860A. I am very happy with my gear (previously I had McIntosh C2600/MC302 with Moon 280D) and it’s transparency, detail and weight.
I have heard 860V2 and it’s amazing when I heard it in a $100k system (Linn’s new DAC, Moon 700 series preaamp and Wilson Sasha DAWs). I’d be inclined to by the 400Ms because you get 100% in year 1 on a trade-up and 75% on year 2 on a trade up.
Let us know how it goes.