Magneto is an effect module designed to receive synth level input signals. A standard guitar’s output is typically about 18dB too quiet to work well with Magneto and the input impedance does not match well with a guitar’s high output impedance.
Plugging a guitar into a boost pedal that can provide at least 18dB of clean, high-headroom gain will work to raise the input level and provide better impedance matching for Magneto’s inputs.
We also have our own module available called AA.1 to interface guitar level signals with Magneto and other Eurorack devices. It has at +18dB amplifier to boost your signal for Eurorack and an -18dB attenuator to lower your Eurorack signal down to instrument level for guitar setups.