Could someone explain quality sound in preamps?

As I understand it a high quality preamp should let a signal pass through it without adding any of its own "sonic
signature" or coloration. So why tubes or solid state? Why not just passive preamps? In other words can a high quality preamp add a coloration that would be considered an improvement in sound quality? I'm thinking of getting a high quality preamp but a little confused. I would like to smooth the sound ever so slightly. Anyone? Thanks.
Many people have had great success using passive linestages. But, you have to be very careful to get good results. Ralph Karsten offered the following thoughts about the role served by a preamp (linestage), and some thoughts about passive linestages, in an earlier thread:
A line stage should perform the following functions:

1) control the effects of the interconnect cables in the system. In general, transistor units do this rather well and tube units do this rather poorly, passive units do it the worst.

2) provide a proper volume control system. This means that the position of the volume control should have no impact on the quality of the sound (a big problem with passive setups and digital volume control systems, a very noticable issue with the vast majority of remote volume control systems).

3) Gain is helpful, but not manditory- a lot depending on the amplifier being used and the efficiency of the loudspeakers being used. Generally moderate and lower efficiency speakers (92db or less) will have some benefit from a little gain in the line section, unless the amplifier has a very sensitive input. If the unit does have gain, that gain must be wide bandwidth (+100KHz) and low distortion.

4) provide an input signal switching capability. This again is not mandatory, for those who only have one source. However input switching can greatly add to ease of use (and if the stereo is hard to use, it will gradually get listened to less often). If input switcing is present, it should be sonically transparent.

Given thes things, ultimately the goal of reproduced music is to sound identical- indistinguishable- from the real thing. To this end the line stage is critical, and this is not about taste, it is about accuracy. Most transistor units tend to impart a stilted quality that is instantly recognizable to the trained ear. This raises the bar on transistor units, leaving tubes to be the main contendors. The possible exception might be a buffered unit with an extremely high quality volume control, but the higher resolution systems usually have no trouble revealing even the relatively transparent buffer circuitry.

What some active buffering circuitry in the linestage can provide is greater flexibility, greater isolation of component interactions, buffering of mis-matched impedances among various other components and among interconnect cables themselves, use of longer interconnects where this may be a need, sometimes improved dynamics, sometimes improved tonal balance (because of control over all of these other interactions). This is all HIGHLY system dependent. While a good passive linestage can be a great solution, the bar is raised for excellent execution within a given system implementation. If you are prepared to pay attention to these things, a passive might be the right solution. If not, a good quality active linestage takes care of these issues for you, but adds the challenge of some active circuitry that will have some sonic signature of it's own to contribute.
Thinking back over the preamps I've had, the main terms that come to mind are: noisy (controls on Dynaco PAT4); blurry (SAE MkIII & P102); dark (bass on Forte F44); natural (Threshold FET ten/hl); and fast (Threshold FET nine/e). In fairness, I enjoyed them all. Each was an improvement over its predecessor. More to the point, often a preamp's real attributes did not emerge until it was replaced by another.

The bad news is you may never find a perfect preamp. The good news is almost every component has some special signature quality. For example, the Threshld FET ten/hl offered a warm and natural tonality I've not encountered elsewhere. In my system, for the first time ever piano sounded right and real. However the ten/hl was a bit shy in high end extension. In comparison, the FET nine/e has high end extension to spare, is amazingly fast and rhythmic, but loses a bit in the tonality department. Must I sacrifice one quality for the other? I probably won't know that for sure until the day comes I replace the FET nine/e. Until then the most realistic thing I can say is I'm still exploring its ultimate character.

All things considered, how your preamp behaves may have a lot to do with the rest of your system. Masking and revealing happen all along the signal chain. Consider that sins of omission may be more forgivable than sins of commission. Ever so slightly smoothing your sound may make you wonder why your system sounds dull. Or maybe not. Anyone could tell you all day how different components worked for them, but the true test is slapping them into your own rig to hear how they do with the tunes you like to play.

In my book, reputation, reliability, serviceability, industrial design, feel, looks and even color (hiho silver!) run right up there with sound. Thankfully, good or prudent taste in non-sonic areas are useful indicators of care and quality in sound. Sure we'd all like to get the be all, end all stuff into our systems. But for better or worse, tastes and perceptions evolve as we learn and grow.
Get a Joule-Electra LA-150 and live happily ever after. I've been using a superb passive for about 2 years after trying 4-5 $2,500.00 - $8,500.00 tube and SS actives. The Placette Passive beat 'em all in transparency and realness. Thought I'd give the J-E a try after some discussions with those who liked it. It's brought a life to music that nothing else in my house has. I'm now trying out the Placette as a better remote volume control than J-E's but this tube based design is musical, musical, musical!
Rushton, Rockvirgo, and Tomryan,
Thanks for your responses. Much appreciated.