according to the manual, "The TRS connection in the combo jacks for inputs 3-8 bypass the microphone preamp entirely."(page 36)
it also say's this: "Return 1 and 2 (Line Inputs). These are the line-level inputs for channels 1 and 2. Their gain structure is identical to the 1/4” inputs for channels 3-8 and are typically used for signals not requiring preamplification (CD/MP3 players, effects processors, etc.)"(page 33) http://www.presonus.com/uploads/prod...sManual_EN.pdf
so the answer is yes, you can bypass the preamp on all 8 channels using the two insert returns for the first two channels and the 1/4 inputs for channel 3-8.