1. the distance from the pick up to the strings is very important because of the magnetic field created by the pick up. when the strings move within this magnetic field it creates an electric current, this in turn creates the sound you hear. if the strings are too far away from this field, or out of it entirely, then you'll have weak output, and if they are too close then it will interfear with the string movement, thus making it sound out of tune, or like complete ****.
2. impeadence is kind of hard for me to explain, think of it as noise. the less noise the quieter, thus more noise = a hotter pick up, if someone else could explain this better in reference to pick ups please due.
3. line level is a different level of output. it is a much weaker signal, like the rca cables on the back of a tv. they are red and white. well these are also a much cleaner signal, and with it being quieter also, it makes it possible to plug directly into a computer sound card and not be distorted, or too loud for the input.
4. earthing or grounding the strings is not dangerous, in fact if you didn't ground the strings via the bridge it would buzz you right out of your room. grounding makes the noise go away, and shielding makes the quiet quieter. they are refering to a malfunctioning tube amp sending the electricity to the ground of the guitar instead of its cable or chassis ground. electricity fallows the path of least resitance, this being you, and ZZAAPP!!! so if you were to put a proper shield or a capacitor in the input cahin of a guitar, it would protect you from such an insident.
5. a preamp is a way of controling the output of an electronic source. it is like your guitar amp there is a pre amp, which controls the EQ, and the gain, and it make the intial output, then this signal goes to the power amp, which controls volume of the amp. so if you had an active pick up, you would need a way to make the very quiet output of the pick up louder and you wanted to adjust or set the EQ levels then the pre amp is the way. the pre amp is just the first amp, hence PRE-amp.
and the reviews are mostly by EMG snobs, they are the same people that say "Vintage" guitars are better because they were first. the HZ4 and the 81 sound very similar, but the passive HZs allow you to wire it however you wanted, ie coiltap, out or in phase, parallel or in series. however the passives are still passive, so they are a little noisier, but propper shielding will make this an almost moot point.
does this explain anything?