Can a direct CD to amp connection degrade sound?

I've had a Cary 300/200 for over a year and always ran it thru my Rogue 99 pre amp. A week ago I tried going CD to my Rogue Zeus amp direct. Many ads suggested that with a volume-control CDP, a pre was a waste. Sounded right on paper, sounded very wrong in person. The sound without the pre was flat,tinny. There was no air around instruments. What happened? How can a pre improve a CDP's signal when the whole point is to leave the signal unchanged? Has anyone tried it both ways and compared the results?