Gain and linestage

In short it works like this.


If the poweramp needs 2v in to get it's rated full power, and you only have 1.5v max at full volume from whatever is feeding that poweramp, then your .5v short of getting the amps full power. But then the speaker has to be able to take it also.
Active Preamps with "unity or low gain", Passive Preamps, as well as source direct, "can" do this.

But if the poweramp only needs .5v in for full power out, then anything before it with extra gain (more than .5v out) is not usually needed.

Cheers George

Good enough.

If you hooked up your amplifier directly from the output of a CD player or DAC you would blow out your speakers. A CD player puts out a signal of  2 volts max  and a power amp reaches full power at about 1.4 to 2.0 volts. That said, a preamplifier with (any) gain would make things much worse. That is why preamplifiers cut the voltage from the source through the volume pot and then gain up that divided voltage out to the amplifier. 

An amplifier can reach full power from a CD source with just a linear volume pot before the amplifier. The only way a preamp can impede that is if it divides the voltage too low through a poorly designed volume control and that has nothing to with preamp gain or amplifier power.