So the channel imbalance occurs at any volume control setting, and only with low impedance headphones? Hmm.
One last thing to check. Make sure the output resistors are all still 0.47Ω. Most DMMs can’t get an accurate reading of such low resistances, but the readings shouldn’t be too far off, and should be consistent amongst each of them.
If this still doesn’t reveal a problem, then I’m out of ideas...