So, yes, unity is that. And unity is helpful if you don't want to use any gain from the deck itself (for a consumer product, the M10's internals are VERY quiet). If you're using a preamp. the idea has always been to set unity on your recorder and ride the gain on the preamp.
That's all good, but which setting
sounds best?
Perhaps more important, at which settings do we notice a deterioration of audio quality? Whether that is something that can be readily measured, or whether it is subjective.
I persistently reported here that the R09 distorted at settings below 8. Some argued with me about it. It seemed to take quite a while until it was accepted as something to be strongly avoided. The distortion was a crunching on intense bass. It probably isn't something that could be easily measured by normal tests.
I was getting out to a lot of shows back then. And I frequently ran the r09, 722 or Microtrack in parallel. That made it easier to chase down differences in performance.
It is something I am curious about with the m10.