All preamps sound different,Yingtonggao this is why you've bought hundreds?? they do not all sound like straight wire with gain (uncoloured) otherwise they would all sound the same and there would have been no reason for you to buy hundreds.Another thread, still missing the point.
As they are all coloured in some way and not true to the sound of the source you are looking to find the colouration you prefer. Just these two statements of yours have basically said to me just that.
People don't just listen to a DAC but a system of components. Every component is colored and has a sonic signature. They mix and match to get proper synergy. It's possible they don't like sound of DAC with just straight wire/gain but only in a system because of synergy. I'm sure you can build a very nice system with a passive pre. There's more than one way to skin a cat.
With all the hype, I bought a Lightspeed Attenuator. It's good but NO comparison to my active pre. The active has superior imaging with well defined edges, 3D, dynamics, vivid, color ...
My active pre is relatively expensive, it's GREAT value with the quality of sound it produces. Same with my speakers, relatively expensive but I prefer over speakers costing 2X+ ... Components don't have to be low price to be good value IMO.