Reg...
The use of DI comes from the concept of getting the guitar's pickup sound directly to the console, bypassing the guitar amplifier completely. The intention after having recorded that way would be to re-amp that signal to restore the "tone" of the guitar that is lost not having gone thru its amp.
The reason is that the tone directly off the pickups is not generally usable in many cases - it simply doesn't have the coloration it normally would have. This is ok though, because you're going to be "coloring" it by running the recorded signal back thru an amp, or a POD or some other guitar processor that will restore the typical tone.
SO.......... if you then use a preamp to try and "color"
a direct guitar signal, you are still not acheiving that amp-driven guitar tone (because a mic-pre is not designed to do that) but yet you're changing the signal off the pickups in a way that may affect how easy it is to "re-amp" that guitar signal after the fact.
Mic pres advertised as instrument pres is not wrong or false advertising - but the instruments they're talking about is keyboards and the like - ones that don't require an amp to "give it the tone".
The guitar is different in that there are really 2 stages to the sound - the signal off the pickups, and the end resulting sound of THAT signal being run thru an amp or POD-type device. In recording using a DI, you simply want to accurately capture that first stage only - signal right off the pickups. The second stage gets applied later. I suppose the next question might be why? This technique allows you get the guitar performance recorded first, then the actual tone can be worked out later, during mixdown. This allows for great flexibility because right until mixdown you can select an appropriate amp tone to suit the song, without having to have the guitarist around!
If you had originally recorded the sound with the amp and at mixdown found the sound inappropriate, you'd have no choice but to re-cut the track.
The downside of the DI technique is also its advantage - flexibility - if you defer enough of these sonic decisions until mix-down, you can easily lose perspective of what the song originally was supposed to sound like in the first place. There IS something to be said for finding the guitar tone you want and simply recording it right then and there (without DI) - and NOT waiting unitl mix-down to decide the tone.
Yikes... long thread!
Hope it explains it though!
Bruce