I had tried both ways. While my preamp (VTL TL 2.5) was in service, I was forced to feed the Cary Audio 303/200 CD player directly to my ML 23.5 using the built-in digital volume control. Music sound up-front and I enjoyed it for a little while. However, I cannot say I could enjoy the music for a longer period of time.
Once I got back my preamp, it was a completely a different story. Music was laid back, but depth and left-to-right imaging got more defined as opposed to direct connection. VTL 2.5 is a tube preamp (2x12AU7 and 2x12AT7) and male/female voices were so much better.
I have two DACs in the system (AR DAC2 and DAC3) feeding Cary Audio SLP 03, another tube preamp, and I don't think I will ever go back to direct connections. Preamp is a must have in your system.