The auto volume is supposed to eliminate the difference in volume between BAL and ACGI'll test out the Auto Vol.
Official explanation of the two Balanced Drives:
"Balanced output via the 2.5mm 4-pole jack supports the more standard balanced (BTL) drive as well as the more esoteric Active Control Ground (ACG) drive. ACG drive is a different form of balanced drive where the amp’s cold side actively drives to create an ideally stable GND. Power gains and increased sound separation increase stability for a clean signal and greater sound clarity, especially compared to unbalanced drive."
So both are Balanced and I don't think it's really about headphone pairing. Not using Auto Volume and using Bal instead of ACG so increased vol can net better results for hard to drive headphones like my Amiron, will test out later
Turn it on and purely compare the different in sound effect. That is interesting.
BAL provides powerful sound (double output power), ACG provides similar audio signature (and output power) as unbalanced
ACG actually doesn't look attractive.
But the difference between BAL and ACG is significant in some headphones.
For example, my CKR100 with HDC212A cable
In BAL mode, it sounds like two circles centered at my left ear and right ear. Headroom is extended from the centers. Sound is rich and headroom is wide.
While in ACG, it is a single circle centered at the middle of my head. Sound is less rich and headroom is slightly narrower.
For BAL, although the headroom is wide, it sound a little bit strange when the environment is quiet, as I can clearly notice the two circles.
In this case, ACG sound much natural to me.
Therefore, I use BAL when I am outdoor, and use ACG when I am at home.
So this bring me to the "headphone pairing"
Some headphones are designed to center the sound. BAL can change such audio signature artificially.
This could result in an odd sound. These headphones do not match well with BAL