I have got a Taylor 312ce 12-fret. Since I could not find any acoustic multi-effect processor which has no pre-amp integrated in it (which is also something I do not get), I now got a Trace Elliot Transit a. The problem however is that it amplifies the quite low noise of the guitar itself such that it results in a constant, quite high, noise level. You can find an amplified sound example here. First part is without pre-amp, second part is with pre-amp. All equalizers are set to flat. You may ignore the hum, this is another issue (tackled in a separate question).
Edit: Setting the external pre-amp's treble control to -15db nearly eliminates the hiss. Turning the treble control on the guitar's preamp to a maximum however does not reintroduce it. Any ideas? Is this normal preamp noise?