what's 'Active preamp' vs 'Passive preamp'?

Active preamps have the ability to add gain (amplify) to the incoming signal. The vast majority of preamps are active. The tip off is whether it has a power cord.

In general usage, a passive preamp is one that does not have an amplification circuit. I.e., its volume control acts only to reduce the volume, starting from the full input signal from a source. Most sources output at high enough a level that they can drive a system pretty loudly without any amplification at the preamp stage. As a result, all the preamp really needs to do is reduce the level to a loudness that the listener wants to hear. This can be done in a number of ways without requiring a power source inside the preamp, including using a variable resistor in line with the signal or using a transformer.

Of course, there will always be instances when the listener may want to hear something louder (perhaps because his speakers are very inefficient or because of a signal level-diluting impedance mismatch at his amplifier), and this would require that the preamp increases the level of the signal using an amplification stage. Such a preamp would generally be called an "active preamp" because it would have an active circuit that would require a power source, usually wall current or in some instances battery power.

Of course, a phono preamp would have to be active since phono cartridge output levels are usually quite low and their signals are not recorded with a flat frequency distribution and thus require RIAA equalization, but that's a different subject.

Reading further in to your question, I agree that all preamps are passive but only in the sense that they are not intended to drive speakers directly. "Amplifiers" or basic amplifiers are required to drive speakers. (When the preamp is built into an amplifier, such a unit is called an "integrated amplifier.") Your use of the word passive is in a broader sense than the audiofile use of "active" and "passive" when referring to preamps.

The general definition of 'active' and passive' is similar to the audiophile definition.

Passive components are resistors, capacitors inductors and transformers, none of which requires a power supply in order to function. Passive preamps are made of these devices.

Active devices include transistors, integrated circuits and vacuum tubes (among other things) require a source of AC or DC power in order to function and their inclusion in active preamps is what is indicated by the AC cord.

Actually, you can make the volume attenuation even simpler - avoiding a "box" and having to add another set of interconnects. I have a pair of "shotgun" attenuators which sit between my interconnects and my amp.

Attenuator in my System