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



In my experience a pre is a clear improvement.  I assume there are situations where it isn't but the first time I put one in my system I was close to shocked.  I believe I was running a pair of Classe CA-200s bridged mono in addition to a pair of Infinity Intermezzo 1.2 subs.  All out of a Benchmark DAC1 with the balanced out to the amps and RCA to the subs.  This was around 2008.  I bought a Bryston BP25 used for a grand.  It made the overall presentation a ton more forceful and distinct.  Bass was more distinct and the mids gained a lot of texture.  I assume all the interconnects and multiple components were taxing the DAC output capability beyond what it could keep up with.  The system may have lost a trace of transparency but overall it was much better.  

One day I expect I will have a combination DAC/Preamp form factor from MSB or DCS and do away with the separate pre, dac, streamer, etc.