So I actually did this test described by
@staticV3 below, using REW after seeing it on the HD 560S thread:
Easy, just listen to sine waves at different frequencies in Mono and adjust the L/R balance until the tone is centered.
Do that at 50, 100, 150, 200, 300, 400Hz..... and you have a channel imbalance vs Frequency graph.
With right channel set 3.6dB lower than left channel, tones stay central up to 6300Hz
I then play tones jumping up 1/3 octaves.
Then at 8kHz the tone shifts to the right
At 10kHz the tone goes left
At 12.5kHz the tone goes center again
Yikes! So simply reducing one channel by fixed amount is not enough.
The drivers have non-linear behaviour
This is bad.
I thought i could probably live with the headphones with a fixed level gain applied but its not good.
At 7.5kHz specifically the tone is FULLY right - even though REW is outputting left channel only and I am mixing left channel from REW to both left and right drivers.
So the left driver basically doesn't even work at 7.5kHz !
7.5kHz tone on my HD 560S plays centrally - so it is not my hearing