Preamp - what's the purpose?

Intentionally dumb question...

I've heard various 5-15W tube amps in my room. EL84, 300B, etc. They all have input stages and the output stage. I send them a line-level signal from a DAC.

Sitting a few meters away from my loudspeakers, the first watt alone gives me roughly 80db of volume. I think these amps are biased to expect the line level signal directly. Why wouldn't the designer do that?

So what's the point of adding a pre-amp? Why do people do it?

thanks in advance



@clustrocasual -- "I wondered about people who use it [a preamp] to color the sound..."

Its been explained that "colored sound" can also happen when there is a mismatch between the source's output and the power amp's input. 

You can't just assume that all line level sources are capable of optimally driving a power amp. Sometimes that's true, and sometimes its not.

Never been a fan of preamps. The main course is the amp, preamp is like adding salt n pepper.

The signal coming from your source and being fed into your power amplifier needs to be in the sweet spot of your amplifier's sensitivity.  Meaning that you are well above the noise floor of your amplifier, but not approaching the output clipping level of your preamp or DAC device.  Here is an interesting paper on the topic:   Gain Structure 101
If you wish to have a streamer with a built-in DAC you need to choose one that has an excellent analog output stage, offering sufficient gain and low noise. A good example of this is the  miniDSP SHD.  Many of our customers who have adopted the SHD to gain the benefits of Dirac Live initially had very expensive preamplifiers in their systems. Given some time, in most cases we've seen these preamplifiers removed with improved results.