The m500 doesn't have high gain mode. It has full discrete amplification and output with digital volume control. It is not the most powerful amp out there, far from it, but it is very clean, and the 40 steps on the volume control are enough for using either high sensitivity iems or high impedance headphones.
Currently I seldom use the m500 as an amp, except for iem or headphones that I can only connect in single ended mode. Been using it yesterday with my 150 ohm Sennheiser hd565 ovation, and it's pure bliss at around 17-22 volume. With my AKG k701, I use 14-20 volume. The Grado 60e will play at about 10-16. The AKG K340, as mentioned, will only start to blossom at 28...29.
Most of the times I use my AKGs at balanced mode, as they are modded that way, connected to a loxjie p20 using the balanced outputs on the m500.
As for MQA files, I stream tidal through mConnect to the Raspberry Pi 4 running MoOde Audio. This upnp solution allows MQA, and it shows the correct sample rates and MQA icon in the m500's display. What I could never achieve is the purple dot using this streaming method. Don't know why. When I tried Roon with the same setup, I could manage to get the purple dot. But the need for a core running all the time and the price, it ruins Roon for me.
It would be nice if someone could figure this purple MQA dot out. Why and in what circumstances does it shows? What does it really mean? And the green and blue?