All preamps will add a coloration. Recording Engineers know that instictively when going to grab Neve, API, SSL, Manley, etc. Most peoples experiences with passives aren't that pleasant due to impedence mismatching and the neccesity for high quality internal components. Done right it works well. My home system still uses a "dual mono" PS audio passive that I keep going back to again and again for it's sheer pleasant musicality.
If you want great sound with little thought or effort avoid the passives and pick up a Manley.
If you want great sound with little thought or effort avoid the passives and pick up a Manley.