From the manual "When a device is plugged into the Line input, the mic preamp is bypassed, and the signal is routed to the instrument preamplifier stage." The line inputs on 3-8 are not designed for instrument level sources. To test, I tried plugging in a guitar directly into channel 3 (mic/line) on my Studio 192 and the signal was extremely weak. While turning the gain all the way to max did boost the signal, it was still very quiet and unusable. Plugging the guitar into input 1(mic/inst), I only needed the gain at about less the half to get a strong signal. I would guess you would need to get 4 direct boxes for the other 4 outputs on your hexaphonic guitar to get good signal levels.