As noted above, preamps are next to unneeded today, due to the high level output of most components which can drive power amplifiers directly. Historically, preamps were "control" amplifiers, which not only amplified phono signals and allowed source selection, but also held considerable signal processing capability: tone controls, loudness and other contour sound shapers, balance and more. As signal processors they reached their zenith in the mid to late 70s with models like the Sony TA-E8450 and models from Soundcraftsmen, SAE, Carver/Phase Linear and the like offering enormous signal processing (even huge graphic or parametric equalizers, noise reduction, dynamic expansion) to try to make up for the limitations of the recording technologies of the time. Of course, all that extra circuitry added colorations and noise of its own. The trick was to give as much flexibility as possible without dirtying the underling signal too much.
But of course, technology improved. The late 70s / early 80s marked the advent of the bare-bones preamp - first by boutique manufacturers and then by the Japanese mid-fi producers; manufacturers cut out tone controls and minimized circuit paths once it was realized that EVERY component changes the signal. Up until then, it was widely held that passive components (resistors, capacitors, inductors) had no intrinsic sounds of their own. We now know that idea is completely wrong. Perhaps the death-knell for the full featured preamp was the advent of the high signal level CD and the decline of the low-level phono source. One could run CD directly to power amp, ensuring that the "perfect sound forever" of the CD was unadulterated.
With the resurgence of vinyl (at least among people of Audiokarma and the like,) there is again a need for low level amplification, but the current generation of phono preamps is very quiet, very true, and requires minimal, if any, post-signal processing - provided you have a good signal. Sadly, the popular recording industry often squanders the advantages that current technology offers (the "loudness wars") and there is again interest in signal post-processing. Check out the higher-end used dbx companders from 20-30 years ago - today, 4bx units regularly go for over $400 and 5bx units routinely sell for $700-1000 - as much as or more than when they were new. For those with purely digital interests, there are any number of computer based signal-processing suites which offer far greater signal manipulation - but done in the digital domain, where the introduction of noise and unexpected anomalies can be controlled/minimized.
Again, as noted in previous posts, changing a preamp can make a marked difference in the final sound coming from the speakers. I disagree, however, that the power amp adds the least to the signal. It SHOULD be neutral, but if you listen to lots of power amps, they all (especially vintage ones) sound different too!
