0dB gain question

What is the difference between ...0dB gain in active preamp and ...0dB gain in passive. Another words what is the difference between passive attenuation and active.
I'm tech-oriented so the input from engineers and professionals will be appreciated.
Marakanetz, gain is the opposite of attenuation. To have gain, implies amplification, and this type of system is in "active" preamps only. Passive preamps are really only input/output switchers with a global attenuator which can only attenuate(reduce) the sound level. In practice, the volume control(attenuator) simply lets more of the source signal through as you turn it up. "Active" preamps take low level signals and amplify them up to more useful levels. The confusion you have may be related to the markings on volume controls, whereby steps may be incremented in apparently different values when using passive or active, and amplification is/is not taking place.The 0db gain on either one should signify the position where no attenuation or amplification is taking place. On a passive unit this should be at the extreme right(full up) with the lower increments marked in negative(-) values. On an active unit the 0db point location may vary from unit to unit depending on the design, but there will also be positive (+) values for volume steps above the 0db mark.
The real issue is that the active preamp will have a constant, low impedance output. The passive will usually have a higher and somewhat variable output impedance.
...so theoretically saying that passive preamps are more volume position depended than active ones and best performed onlyat high volume levels?
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!