As with most things audio, there are many opinions regarding the sound quality provided by active and passive preamps, including the in-line attenuators discussed above. In general terms, a preamp accomplishes three things, a. switching between sources, b. attenuation (or gain-if active) of the source output signal, and c. impedance matching (or in some cases, mismatching). Item c (in addition to output levels and sensitivity), is a reason that passive preamps do not work well in all systems. If you are really interested, or really bored, there is a ton of reading on those subjects in these threads.
