you are correct in thinking that all you wiil gain is more distortion.
I'll word this as simply as possible.
Your amplifier can put out only so many watts without distorting.
Depending on how "loud" that signal coming in is, it may be reached at a low setting or a high setting on the volume control.
To reach the same output level, a low level signal coming in will have you to turn the volume control up higher than you would for a high level signal, which will need the volume set lower.
In either case, the amp is still capable of the same thing. It's only a matter of how you have to set the volume control before you run out of clean power and start distorting.
Incidentally, those "RCA signals" you refer to are called "line level" in this field, and generally range from 1 to 10 volts with very low current.
You saw the spec sheets on the preamps. It's there in black and white.
Why not ask the engineers why they designed them with so much output potential?
Obviously, the engineers that design them had their reasons but, as the preamp specs show, this voltage can be well over one volt. Maybe because they believe that it's better to have more than you need as opposed to needing more than you have? Maybe different power amps have different sensitivity and gain?
Of course, when you get to designing and manufacturing your own products you can do it your way and prove yourself right in that instance but, until then, have fun dealing with the reality that this line level signal can be well over one volt.