I had a dac3 hooked into a s300, it did not sound etched or harsh. I also had a benchmark before the dac3 and chose to keep the dac3 for this reason. I also had a cary slp-88 preamp in the chain for both. While playing around with taking the preamp out of the chain, I found the sound to be clearer, smoother, and more detailed with just the dac3 and the s300. Then a few months ago, I got a Jolida 502brc. It bested the s300 in every way possible.
So in adding tubes to your system, you could probably take the edge off with some warm sounding tubes, ( red base 5692 would have done it in the Cary )but it may depend on the quality of the tube component ( I am aware Jolida is not the be all of tube amps, but the slp-88 is entry level for Cary ), and the tubes you use. It could just be the sound of the DAC1
So in adding tubes to your system, you could probably take the edge off with some warm sounding tubes, ( red base 5692 would have done it in the Cary )but it may depend on the quality of the tube component ( I am aware Jolida is not the be all of tube amps, but the slp-88 is entry level for Cary ), and the tubes you use. It could just be the sound of the DAC1