I guess no preamp is better, unless the preamp is really good! :-)
That has been my experience as well.
For a short while I tried my Berkeley Alpha DAC Series 2 direct into my Pass Labs amplifier (with Magico speakers) and found the sound was super detailed, clean, and dynamic. I was very impressed.
I then tried a SimAudio 740P preamp in my system and much preferred the overall sound with the preamp present. I likely lost a tad bit of detail with dynamics and timbre being similar, but the BIG difference was in the explosion of soundstage and space between the instruments and vocals. The added front to back depth and 3d like imaging made me have no desire to save the not insignificant price of the preamp and go back to using the DAC direct.
Using the DAC direct was like a 2d super detailed presentation, where the 740P made it sound 3d and more like the artists were physically playing in my listening room (albeit maybe not in a recording studio...).
I've heard very big differences while trying various preamps over the years and have no doubt I would prefer the sound of a great DAC like the Berkeley direct in many of these cases; but I'm also unfortunately sold on the value of a great preamp in making one's system sound its best, even when you have a single digital source like me.