Its line input is fine. But the external mic input seems to have the same issue. So I guess that there may be bad components in the mic related circuit, such as mic amps or switches...
I got a brand new PCM-M10 several months ago. I just noticed that its channel balance is not set correctly. The right channel is always higher than the left one by about 3db. It happens for both internal and external mics. Is there a way to adjust the balance? Or is it a known bug?