ight, he doesn't specify that, but a slave output is usually a line level signal, not an instrument one.
Line levels are 0dB, +4dB nad you can go much higher, +20dB, in pro_audio mixers.
-20dB or -10 are considered instrument levels, typical in a guitar or keyboard. Some keyboards can go higher, at 0 or + 4dB.
Using the balanced input, even with unbalanced wiring/signals, matched better the impedance and level of the signal and has the full headroom a +4dB line level signal requires. In addition you have to consider that a line level can have much wider dynamic excursion than an instrument one. The signal from a gtr rack preamp can have very different dynamics and levels from low to high, whereas a guitar doesn't.
You can some fun connecting a guitar to the XLR, with unbalanced wiring, and you'll hear what I'm talking about.
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