The switch changes the MP1 A and B outs to instrument level (millivolts) to plug into your guitar amp (clean) input OR line level (~1 volt(s)) to plug into a power amp (which is designed to be driven by a separate pre-amp/mixer etc at line level). Your guitar amp has a pre-amp and power amp and it's pre-amp is designed to take guitar (instrument) level signal. If you send it line level, you'll overload the input (badly and may cause damage). If you plug the ADA set to instrument level into a power amp, it won't have enough herbs to drive it and it will sound week, thin, yuk, noise etc.
The new(ish) ADA site (http://www.adaamps.com/index.htm) has suggested pre-amp eq settings in the ADA APP-1 user manual for a bunch of guitar amps (Marshall included) to provide the cleanest flatest response (no add colour) for people who want to run their ADA APP-1 into a guitar amp. These are just as valid for MP-1 into guitar amp. They are on page 15 of the APP-1 user manual (attached), it also includes some other good advice regarding using a guitar amp with these pre-amps.