Correct me if I am wrong. As I see it amplifier gains are used to match hu input voltage. Ideally, your hu input should match the gain level.
Here is what I did with my Orion Xtr 375. I used a DMM to read head unit output at the RCA's with a 0 dB, 80hz sine wave. I read 2.6 volts at roughly 3/4 volume. All settings flat and filters/crossovers off.
My amp has an option for outputs; copy, master, or gain. On the gain setting the output level matches the input. I used the DMM until I read 2.6 on the output RCA. The amp should now be optimized. Correct?