Attenuating a line level by around 20-30 dB to feed an instrument level input should work
It is difficult to give typical levels for things like mic signals as
different types of mic have different output levels and something like loud shouty vocals
into a high output condenser mic could easily give levels more typical of line outputs
whereas distant mic'ed quiet acoustic instruments with a dynamic mic would be much
Speaker levels can also vary from a few hundred millivolts up to 100
volts or more, depending on the volume and impedance involved.
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