The info above regarding the input sensitivity of the power amp constitutes the majority of the answer to your question, but there is one other piece of information that may help you to understand what is going on. There is often a misconception about the "volume control" on the preamp. Volume controls don't actually "turn up" the volume, although that may seem to be the case. Actually, the volume control is an attenuator, which turns down the volume from its maximum output. If you were to remove the "volume control", the preamp would simply operate at maximum voltage output. Hence, the volume control actually decreases the voltage from the preamp's maximum full output.
In many preamps, the volume control is anything but linear. Many volume controls have a rather limited operating range, reaching close to their max output by the time the control knob is around 11:00 - 12:00. If your preamp volume knob must be turned beyond 12:00, it's really not a big deal -- it simply means that the volume control has a more usable "range" before reaching the point of lower attenuation.