Great question. I just assumed they were both wired to the preamp. Looking at the manual right now, and it gives the impression the TRS connector doesn't hit the preamp--the preamp connects to the XLR and has a gain control range of 60dB and a max input level of +12dB, while the TRS doesn't have a gain range number and the max input level is +18. Sounds like the TRS is a straight line input, but I'll check tomorrow if the gain control on the channel is still active.
Thanks for asking this, I've been trying to chase down some crispyness using my old high gain Quad Eight preamps straight into the XLRs and never even considered that the TRS might bypass the preamp.
Presonus, how about it?