Preamp - what's the purpose?

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. 
 

Funny no one seems to remark/notice that if your DAC has sub circuitry, gain, etc it IS a preamp too! 

@mbmi   The BEST  sound  comes from. TUBE PRE and a SS amp. BS. I had a SS amp. A Krell FPB 600 and an ARC Ref  6se . Now I have the ARC Ref 750s mono blocks  with the ARC Ref 6se. Can’t compare. 

I find it somewhat ironic that those purists who eschew the use of a preamp because it "colors" the sound will then seek out amps, cartridges, wires or speakers to achieve the tone color they seek.