Today I read the thread on high gain/low gain (http://www.head-fi.org/t/543242/q-high-gain-low-gain-quality-difference) and decide to try it out.
Turned out my post regarding the high gain/low gain setting for TA-01 was wrong: There was only TWO levels, not three. Therefore JP7/JP8 serve for each channel. If you set it to HIGH/LOW or LOW/HIGH, you will notice the volume difference between left and right. I believe JP7 is for left and JP8 is for right.
Back to the high gain/low gain setting, I heard from Xduoo that sound quality of high gain is better. But their "sound quality" might not what we mean, rather it might just refers to "power/loudness". Ever since I got TA-01, I used HIGH gain setting, today I changed it to LOW, and it's not that bad. Actually I thought I heard more details and dynamic ranges with low gain, as well as more open stage.
However, since I can't do a side-by-side comparison, this is really subjective and might just be some psychological effect.
Anyway, if your cans did not come with high impedance, low gain might not be a bad idea after all. Just try it out and stick with the setting that suits you best.