Here's the block diagram from the DR680 manual. Perhaps I'm reading it wrong. I don't know much about these things.
According to that DR-680 diagram the only difference between XLR and TRS input routing is the input jack itself. Both XLR and TRS go through the same preamp circuit. Preamp line/mic sensitivity switching is made to the circuit via the hardware switch.
If the trs line levels go through the preamps then why can't the gain be adjusted by using the gain knob? I'm not arguing the the point, just curious.
All depends on how they program it. The TRS DET lines (TRS insert detection) of the Zoom trigger whatever logic they program to occur when a TRS plug is inserted into the input rather than an XLR plug. I would seem to perform the same gain sensitivity switching role as the hardware switches on the DR-680, but may also do other things like disabling gain knob adjustment.
What is common to both recorders is that all signals pass through the same preamp circuitry. This is how most all modern recorders work. If it makes you feel better when using an external preamp, think of the recorder's preamp stage in low-gain mode as being an input-buffer stage.