Studio 192 manual, pg. 6:
Line Inputs 3-8: The ¼-inch TRS connectors on Channels 3-8/4 are for use with
line-level devices. These inputs are scaled to accept line-level signals up to +18 dBFS.
Power User Tip: When these inputs are engaged, the microphone preamp circuit is
bypassed completely, and no trim control is available. Typical examples of line-level
connections are synthesizer outputs, signal processors, and stand-alone mic preamps
and channel strips. Use the output level control on your line-level device to adjust its level.
And from pg. 4:
Instrument inputs: The ¼-inch TS connectors on Channels 1 and 2
are for use with instruments (guitar, bass, etc.). When an instrument
is plugged into the instrument input, the mic preamp is bypassed,
and the signal is routed to the instrument preamplifier stage.
Power User Tip: Active instruments are those that have an internal preamp or a
line-level output. Active instruments should be plugged into a line input rather than
into an instrument input. Plugging a line-level source into one of the instrument
inputs on the front of the Studio 192 and Studio 192 Mobile not only risks damage
to these inputs but also results in a very loud and often distorted audio signal.