0dB gain question

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.

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.