You're asking all the right quexstions, Marion. I'll take a stab at answering them. It occurs to me that we could use a primer on this topic -- don't know if anyone is aware of one, but I'm hoping someone chimes in with a good web site address.

"Piezoelectric" pickups (often called "piezo" for short) are pickups that utilize crystals that transform vibrations into electrical impulses ("piezo" is from the root word for "stone"). It is not synonymous with under-saddle, since many contact (stick-on) pickups also utilize piezoelectric elements. Many people use the terms kind of casually, however. Most under-saddle pickups use some variation on piezoelectric technology, although those of more recent vintage tend to use polymers (plastics) in layers. I believe it was the Fishman company that pioneered the use of polymers in this manner.

"Magnetic" and "soundhole" pickups are generally synonymous -- the pickups that people put in the soundholes of their guitars are basically magnets that sense the vibrations of steel strings and convert them to electromaegnetic impulses. It's the same technology that has been in use on electric guitars since the 1920's or 30's, although it has been refined somewhat over the years. Until recently, most magnetic/soundhole pickups did not give a very realistic "acoustic" sound, but in the last ten years some better ones have been produced.

"EQ" is shorthand for equalization, and generally refers to tone adjustments (treble/midrange/bass). "Gain" is not exactly synonymous with "volume," becuase the two words refer to signal boosts at different points. Generall, a musical signal is put through a preamp and then a power amp. The preamp boossts the signal being sent to the power amp, and "gain" refers to how much of a boost is applied to the preamp signal. "Volume" controls typically change the amounbt of amplification that the power amp is applying. [These are rules of thumb; there are exceptions.]

Preamps for acoustic guitars generally (but not always) offer you some amount control over of EQ. they may be in the panel in the side of the guitar (either factory installed or after-the-fact), or in a small version inside the guitar, often on the inside of the output jack installed at the butt-end strap button. These "inside the guitar" preamps often will not give you any control over EQ, because there's no way to make the controls accessible. Preamps can also be "outboard" of the guitar, in a small box that fits in your guitar case, or in a larger box as mentioned previously. Obviously, if they're outboard, you can use them for more than one instrument. One way to do this is to use an A/B box, which is a simple device that allows you to select whether the signal from channel A (a guitar perhaps) of channel B (a fiddle, maybe) is routed to the preamp; you plug both in, and then opeprate a footswitch to select which one is operating.

EMG is a brand name, as are Fishman, Barcus Berry, Highlander, Pick Up The World, Sunrise, B-Band, L.R. Baggs, and others. A line driver is one name for a preamp, and apparently is the name that PUTW chooses to use. A Power Plug is a product name used by one of the companies (I forget which) to denote a preamp that is inside of the guitar and is turned on by inserting a cord into the ouput jack (at the butt-end strap button, generally).

You can put contact pickups on the inside of your guitar, or on the outside. To install a contact pickup, or really any type of internal pickup, on the inside, it is not necessary to take the top off the guitar. Typically the installer will remove the strings and reach inside the guitar (it helps to have reasonably skinny arms), sometimes with the help of a small mirror on a telescoping rod. One can do this, reattach the strings, try it out, and do it over if the results aren't satisfactory. People who work on instruments tend to have some experience with what kinds of placements work best, so it isn't entirely a trial-and-error process, but there's still likely to be some experimenting. The companies that make these pickups also generally have some recommendations. You can run the wire out the sound hole or have it wired to an endpin (strap button) jack, depending how permanent you want the installation to be.

Keep those questions coming! Regards, WS