I dont think the line input is affected by the mic ,low or high settings.......also it is possible that your connector was not fully inserted into the m10 and therefore you may actually have been using the m10 built in mics.That happened to me before actually thata why i mention it.There may be a small last tiny bit of extra push to be fully inserted.Or your battery in the Ubb was dead?
This is correct according to the M10's Manual:
Page 43:
"You can set the sensitivity of the built-in
microphones and an external microphone
connected to the microphone jack of
the PCM recorder. Switch the MIC
SENS(ATT) switch according to the
recording conditions or to suit your
purposes, when the PCM recorder is either
stopped or is recording.
* The microphone sensitivity is set to “HI” when
you purchase the PCM recorder."