Gboren: You're looking at the subject too narrowly
Not all "preamps" are connected to devices that emit high level signals. There are such things as a phono system, which requires gain prior to being fed to an amplifier. Try running even a high output MM phono cartridge "wide open" into a power amp and see what you hear. Obviously, one needs a LOT of gain in this situation, regardless of the amplifier being used. As such, ANY attenuation of the signal would result in hearing "less than nothing".
As far as line level signals go, some sources simply don't output as much drive as an amplifier may require. This could be due to a weak signal being received from a distant broadcast, very quiet recording ( which applies to tapes, cd's, etc..) or simply due to the fact that the source component itself does not have enough gain in the output stage. As such, additional gain / further amplification may be required prior to feeding the signal into the amplifier to achieve the desired listening levels.
To top this off, some amps offer different levels of input sensitivity. While some amps may roar with as little as 750 millivolts ( .75 volts ) fed into them, others really do need at least 1.5 - 2.0 volts for full output. Since i have seen some sources that put out less than 1.5 volts when driven to capacity via loud recordings, this still may not be enough. For example, the Philips SACD 1000 maxes out at 1.6 volts output in SACD mode when a high level signal is fed into it. On music that does not "roar" and a quiet recording, it is quite possible that you might only have peaks of 500 millivolts to 1 volt available to drive your amp with. For some amps, this would just start to "tickle" them and not really be enough to get their "full fledged attention", hence a lack of volume and squashed dynamics.
All of the above situations would be highlighted even further if using long runs of lossy cable, suffered from signal loss due to poor loading ( impedance problems ), and / or using highly in-efficient speakers.
Having said all of that, i do agree that most of the problems associated with using a "passive" has to do with impedance mismatches and / or weak recordings that aren't strong enough to drive the gain stages of a source component to full output.
If one wanted to avoid such a situation, i'd look for a source ( tuner, DAC, CD Player, etc... ) that was capable of at least several ( 3 - 4 volts ) of output. This should be plenty to drive any amplifer / speaker combo to high listening levels. It would do this without pushing the gain stages of the source into compression, even with a quiet recording. The end result would be no need for a preamp, so long as you only had one line level source that you listened to or didn't mind swapping cables between multiple sources.
Those of us with phono systems, multiple sources and "thirsty" amps / speakers don't mind using an "active gain stage with user selectable inputs" : ) Sean
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