Eddy
You can plug the amp directly into the DAC, using the DAC volume control, and not use a preamp. That's the way my shop is set up: PC with volume control, DAC, power amp, speakers. I have not used a preamp for many years.
Your speakers, 6 ohms nominal and 85 db 1w/1m, are pretty close to an ideal speaker for the SET400. Lower efficiency and 4-6 ohms is what the SET400 was designed for. You might be able to get by with a SET120, especially with your listening preferences, but you would have virtually no overhead and you would always be wondering if maybe you were just a little bit under powered. Go with the SET400, it should be a good match.
Dan