I've used both. Some DACs and CD players with volume work better than others. Bidats and Accuphase work well straight in, from my experience. My Wadia did not...too forward and digital sounding. Very transparent pre-amps have their advantages. There are trade offs either way. A good explanantion is offered by Guy Hammel on his web-site, www.placetteaudio.com.
Preamp or Not to Preamp?
Have very expensive equipment, over $50k invested. But found myself strictly listening to digital on the main system. In the past went direct out with Wadia(about 5 yrs ago before the 800 series), and never had enough explosion of soundstage. Tried passive preamp, and just too week for bass. Want to loosen up some money tied up in the system. Using a Reference 2 line stage by Audio Research. Have the AR 600s mono-blocks, and just go with another Wadia digital out?? I could be saving thousands if I dumped the preamp.