Kijanki,
I'm using the DAC1 without a preamp directly to a tubed poweramp and the sound is glorious. Benchmark even recommends trying it w/o a preamp. The DAC1 has a very nice preamp/volume control.
The difference that some talk about is probably similar to when I used the variable output of my Cary308 cdp direct to an amp vs. through a preamp. You get more body but with less clarity and immediacy. I perfered it without a preamp because it had less coloration and was more intimate in presentation.
Anyway, the Benchmarc DAC1 is so clear w/o the hint of digititus or harshness that it excells as a direct connect to a poweramp. Why muddy up its neutral and honest presentation. The highs are so pure they almost sound smooth because there is no harshness to them. It is a real ear opener.