I of course understand what a passive pre-amp is, I currently use one and have had others. However, when you state that it adds gain, guess what, it is no longer a passive pre-amp. Any kind of gain stage makes it into a standard active pre-amp and no longer a passive. If you are using it without gain you are using it passive.
To quote Arek Kallas, one of the true expert designers in the field, offering some of the best passive pre-amps and attenuators in the world, when you add a gain stage, "In fact it will be an active preamp".
Further more, to add gain does require some type of power. It can't add gain (amplification) without it. So in passive mode it is basically an attenuator and to add gain requires some kind of power (it cannot get louder without a power source). Therefore what is being referred to as gain is in fact more attenuation. With a pure passive, at no attenuation the full input signal is being passed to the amplifier, attenuation actually cuts this signal back to control the volume you hear. Usually with some type of resistors put in line. Less volume equates to larger attenuation resistors. The volume control actually moves different resistors in and out of line with the source signal.
To summarize, you cannot add gain (amplification) without power and adding true gain in fact changes the pre-amp to an active pre-amp, no longer a passive. This has been fully described to me by two of the foremost experts in the field; Arek Kallas, owner of Hattor Audio and Khozma Acoustic and my personal friend EJ Sarmento, owner of Wyred 4 Sound (designer and builder of the world renowned STP-SE Stage 2), SST, Carver and others.
