Look into the difference between gain control and volume control. In very, very simple terms, volume effects the outgoing signal but does not have any effect on the signal. It simply makes the signal “louder”. Gain control directly changes the amplitude of the incoming signal and has a distinct effect on the signal. The knobs on the back of your amp are gain, not volume. You can do something called “gain staging” where you do exactly what you are thinking and better matching the volume output to the amp gain and overall volume of the system. The trouble is when you “turn down” the gain knobs you are no longer getting the most out of your amp, plus adding potential noise into the incoming signal via the pot. With the knobs all the way to “max” they are effectively out of the signal path and your speakers are getting all the power the amp has. Play with it, see how it sounds. If you like the result, good to go. Otherwise, you may need to seek out a new preamp that is a better output level match for your amp.huh?
Not knowing the amp specifically i will guess that they are simply a potentiometer (volume control) in-line between the preamp and the input of the amp. yes, they **could** control the gain (the amplification factor) of the amp - but depending on the design this has complications that most designers would avoid due to unknowns in the real world.
So... i'll guess that its nothing more than another variable resistor divider in series with your signal. You therefore have two - one in the amp and one in the preamp. You can turn either one, or both, down. It really doesn't matter. If you turn on all the way up an d"take it out of the signal" as soemone said, you simply must put more of the other volume POT in the signal. 6 to one, half-dozen to the other. Set it so its most usable and move on. There are vastly more important things to worry about.
G