A pre-amplifier is a trivial thing to design to be perfectly transparent to human ears. I can (literally) take $15-$20 worth of parts(and this is at low volume pricing such as available to a DIYer) and assemble a pre-amplifier that is of very low noise and distortion(far below audibility on both counts).
Most decent receivers will have low noise pre-amp outs that will be transparent to human ears. There may be the occasional unit with an audible hiss. The placebo effect may be a common factor, and I suspect is the primary one, in people detecting [imaginary] differences.
I have perhaps the most absurd standards for audio quality possible, and these days I use an old HK AVR55 as my pre-amplifier. Why? It was cheap(found it in Goodwill for $25), it has digital readout 'dB' adjust numbers and is remote controlled. It has no self noise evident. I sold off my Stereophile Class B pre-amplifier a long time ago - the old HK receiver is a better pre-amplifier IMO(the receiver has digital volume readouts where as the Adcom GFP-750 did not).
-Chris
