Kind of confusing here. You say that the RCA's are on the back of the amps ( which would make them "inputs" ) yet you call them "outputs". This could lead some to believe that you were referring to the preamplifier outputs, which .5 V and even 1 V would be quite low. As such, i'm guessing that you were referring to the inputs on the power amp.
If that is the case, the .5V inputs will probably offer the most transparency but make the volume control VERY touchy. Turning the volume up on the preamp just a hair might jump the volume up several notches. Going to the 1 V input would probably put a resistor in series with the signal. This would make gain settings a little less sensitive in terms of having more range on the preamp's volume control but possibly alter sonics a bit. Depending on how they padded the signal and the output impedance of your preamp, either setting could work better than the other.
You might want to try using the .5 input while paying extreme attention to your volume settings on the preamp. If it is too touchy, move over to the 1.0 V input as that should bring things back to normal. No guarantee's as to which will sound better. Sean