Alexandre,
Yes, I had the Pass XP-30 preamp in my system for over one year and I had the Cary SLP-05 preamp in my system for six months. These preamps both improved smoothness and warmth but also added treble brightness. Running directly from the Esoteric K-01 to the amps (Pass XA-100.5, Pass XA-100.8 and Soulution 501) was more neutral and less fatiguing. I'll be adding the Soulution 520 preamp to my system in a couple of weeks and I'll give a full report. But based on dealer auditions, where we added and subtracted the preamp from the system, I'm expecting the exact same results. So why do it? Because for me, I like what adding the preamp does to the texture of the sound but there are definite trade-offs. I can see why some would prefer to run direct without the preamp and I know several audiophiles who do with outstanding results.
Ken