I'm a bit out of touch these days: do piezo pickups still not come with a FET built-in to change the impedance (as standard)?
If so, that's amazing and rather depressing really.
To answer Mr. Teroo's question, the main reason for a DI box is to balance the output from the guitar, so it will work correctly with the input of a mixer. That's why there is a male XLR socket on the LR Baggs preamp - it does double-duty as a preamp and a DI box. The preamp part would be helpful no matter how you amplify it, because of the impedance issue mentioned earlier (it should convert it to a lower Z, cleanly).
To answer Jacob: both piezo pickups and mics convert mechanical vibration into electric signal. Magnetic pickups don't - they use the changing magnetic field as the (steel) string vibrates over the top of them. Guitar pickups (piezo) are best thought of as very insensitive crystal microphones. You get an acceptable output from them because (a) they're piezo (relatively high voltage output, but very low current), and because the vibration when directly coupled to the body of the guitar is huge compared to sound through the air. Guitar mics will usually be fairly insensitive compared to an ordinary mic, but still be prone to a bit more feedback (howlround), because they will pick up a bit from the PA or amp loudspeakers. Usually this doesn't matter though.
HTH, E.
