The pre and post settings for the looper really don't have anything to do with the effects loop. If you put the looper in pre position, it means it comes first in the M13's signal chain, so in essense, when you record a loop, it will be recorded dry. What that means is that if you change the effects after recording your loop, than your loop will be affected. If you put the looper in post position, the loop comes at the end of the M13's signal chain, and the loop will be recorded with whatever effects you have active at the time. This is helpful if you want to record a rhythm track with one sound and then change to a different sound for a lead tone.
So if you have effects in the M13's effects loop, and you move the looper to pre, you could record a dry loop, than turn the effects in the loop on or depending on what you want, and the recorded loop would change. If you have the looper set to post, if you have the effects in the effects loop on while you record your loop, the loop will be recorded with those effects.