This is the explanation of the balance control from the 18NS manual:
"This is not a standard balance control (of course, it
is a darTZeel machine)!
When using the Balance, the change only begins
after about plus and minus 5 degrees of rotation,
ensuring that when you are close to middle posi-
tion, balance channel is kept dead center.
At full right position, R channel is increased by 3.5
dB, L channel lowered by 3.5 dB.
At full Left position, L channel is increased by 3.5
dB, R channel lowered by 3.5 dB.
This feature allows for very fine adjustment, and
keeps global loudness at same level even while
altering the balance control from central position.
User Manual, version 0.93
Page 6 of 10"
I am noticing a difference at the 10 o'clock position which might be where the change kicks in. IMO this means that you can use the balance control to correct an imbalance caused by the room or other cause without compromising the sound.