0dB gain question

No, the passive unit is strictly dependant on the output level of your source unit(CD player,etc) If you have a high enough output from the line level source, passive is fine at any volume level that doesn't exceed your amplifier's input sensitivity specification. If you have a very low output level from the source, then you will need an active preamp to amplify the source to a level suitable for your amplification unit. As mentioned above, the impedance may or may not be an issue, depending on the system.

One fine point on gain and attenuation. To attenuate means to reduce. Gain, on the other hand, does not mean to increase. It can be either positive or negative or zero. A gain of +dB means to increase, a gain of -dB means to reduce, and a gain of 0dB means no change.

A typical specification given for a preamp is the maximum amount of voltage gain it has. For a passive preamp this is usually 0 dB. In other words, when turned all the way up, the output voltage is equal to the input. However, a passive preamp with a transformer can have a voltage gain. This is unusual though.

Kr4 is half right. Although low output impedance for driving the amp is quite important, of perhaps greater importance is the high input impedance offered by active preamps. In a passive preamp the source component is asked to drive the combined loads of the cableing, volume pot and amp input, often a tough job for the wimpy ICs at the output of many CD players. An active preamp will buffer the input offering high input impedance, low output impedance and, if necessary, gain.

Wow! is there any passive preamp with output trany there?
Or maybe someone DIY-designed one already. I definitely want to try it!