Generally, the gain through the processor input of a preamp is fixed and used as a bypass.
The processor input is fixed at unity gain. This means that the loudness of a surround processor will be identical to what you would hear if you listened to its output directly. That is the whole purpose of a processor input. It is technically not a pass-thru in the sense that you are sending the signal through the active gain stages, just at a fixed attenuated level.
For this reason and a variety of others, I would not use the method you describe to compare the two preamps. The sound of the unit plugged into the REF3's processor input will be affected, sometimes for the good:]