Personal Experience, Trying many possible methods DAC to Amps with various Dacs and Various Amps.
Final Conclusion: Pre-Amps is Required.
Some Dacs have Pre-amps built into them like Playback Design and MSB and various others. However much these companies claim their pre-amps are superior to use directly with Amplifiers the final results is never as expected.
The Pre-amp is the heart of the system, all devices are connected to it except the speakers. It either makes or breaks a good sounding systems.
A Tube Pre-amp injects life and soul into the system, iv not had much luck with SS Pre-amps.