First of all, make sure you haven't accidentally engaged bass roll off on the R09HR (Low cut switch on the back) or on the preamp (the switch is inside the battery compartment on mine). You will probably never want that with cardioid caps.
To answer the original question... The optimal setting is probably to get the ST9100 set so that you can get the right levels with the R09HR set to "mid scale". What's the gain range on the R09HR? 0-80? So the ideal would be to have it set to 40, and most of the gain coming through the ST9100. In my case, I have the original R09 which is 0-30, so I'm shooting for 15.
That said, if you move the gain knob on the ST9100 you might get a lot of static type noise while you are cranking on the knob (at least on mine I do). So the objective is to put that in about the right place, leave it there, and then go up or down a little on the R09 to fine tune. That's what I do.
It's generally said that "unity gain" on the ST9100 is at about 11:00. So below that you are reducing the incoming signal, and above that you are adding gain. If it's a loud show I've learned that I'll basically set it to 12:00, at which point the Preamp is acting just as a battery box without adding much gain. If it's something like a quiet acoustic thing, I'll just crank the St9100 all the way up.