When I originally got my R-09 and ST-9100, both nearly simultaneously I might add, the consensus seemed to be that the unity gain is at 8 so that's what I used. I've now taped 5 very loud metal shows (2 up front and close to PA's, rest at soundboard), 2 semi loud hard rock gigs as well as 4 quieter acoustic gigs. All but of those shows were recorded with R-09 set to 8 and the ST-9100 set to max. The one show I used different settings was the last show I taped, where I had the R-09 gain set to 13, it was also a very quiet acoustic show. The very first gig I taped with it, a Chris Cornell gig, which was moderately loud show I taped at the soundboard. That was the only show where I turned the gain down a bit and in the end I believe I still had it at around +16-17dB. Never had any clipping issues and from the looks of it, it never got even close. For microphones I've used STC-11's for all the abovementioned shows.
Before the year is over, I'll be taping Motörhead, notorious for insanely loud volumes. I suspect everything will still go pretty much the same as before and that it wouldn't overload the preamp, nor the recorder.