I was going to try using the mic sensitivity set to high since the manual doesn't seem to say that the sensitivity setting applies only to the internal mics.
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BTW - right now the mics I have are Audio reality and the battery box spec is a 150Hz high pass filter.
I'm also a recovering minidisc user and by habit I still go mic-->battery box-->Line-In on the PCM-M10. Works fine for me. Guysonic knows a hell of a lot more than I do about audio and electronics, but my amateur thinking is that going through Mic-In adds gain from the built-in preamp, and at a loud concert I don't need gain--or another processing stage.
Someone should correct me if I'm wrong, but as I understand it Mic Sensitivity governs only the Mic-In jack, not Line-In--High Sensitivity raises the gain on the preamp behind the Mic-In jack.
All mics have a different output, but my go-to pairs (two versions of Church Audio, one ancient and unknown, one CA-14 omni), through a Sound Professionals 12V battery box into Line-In, usually call for 5 or 6 to make the green light start to blink when things get loudest--my rule of thumb . Since Mic-In has a preamp behind it, the gain on the knob would be lower through Mic-In for the same result--and different if you have the sensitivity at high or low. You'll just have to find the sweet spot for your mics. Try recording your stereo through Mic-In and through Line-In, getting approximately the same levels by turning the gain knob. and see if there's any appreciable difference in the sound quality.
I also note that you are using a high pass filter. That was helpful at times with the MD because big bass could really freak out the MD. But you definitely should experiment with turning the high-pass filter off or using the 30Hz filter on the battery box. The bottom key on a piano is 27.5 Hz, and rock bass goes lower, so you're really missing something with the bass roll-off at 150 Hz. Especially if you are going battery box to Line-In, the PCM-M10 can handle those big woofers, and you will get much fuller-sounding recordings.