Any preamp will add frequency response anomalies and distortion. You
may prefer that sound to the unmodified signal, but it is good to be
honest (as most of the posters in this thread have been) about what is
actually going on.
One of the most misunderstood tasks of any good preamp line section is to control the interconnect cable. This reduces coloration, which is often heard as an overall thinness and loss of bass impact.
Passive controls often have the issue of effectively raising the impedance of the source by putting a resistance in series with that source. This can often have the effect of acting as if a smaller coupling cap is used in that source, causing a loss of bass impact.
A buffer for the volume control (at the very least) is required; that buffer also has to control the interconnect cable, so in practice many preamps reduce the colorations described in several previous posts, with a much smaller coloration introduced by the line stage- a net improvement overall.
