The R-05 is not discussed much here since most tapers prefer the Sony M-10, so it may be hard to find the unity gain. If the record level control goes from 0-100 like the R-09HR it is very likely unity gain is around 40 though. It reported to be about 38-40 on the HR.
Set the recorder at unity gain to start and set the preamp somewhere between 75% & 100% so that you get good levels.
1) If what you are recording is very loud you need to be sure the CA-9100 itself is not clipping if you have it set near 100%. It has a clipping light that you can check. If it is doing more that lighting up briefly I think you need to reduce the pre's gain. I'm not real knowledgeable about the clipping light though. I usually have the pre set at 50% or so for loud stuff and never even have to look at the light.
2) If you are recording acoustic or quiet sounds, set the pre at 100% and use the recorder to add gain until you get decent levels.