I run 6x MP40s on my 160 in-wall. Couple of tips but your mileage may vary.
1) Turn the controller off before attaching the wet side
2) Rotate the wet side counterclockwise (CCW) a few rotations to float the head over the center of the dry side. Don't go CW or you will likely detach the guard.
3) Turn the controller on at a medium output. If you have noise, slowly rotate the wet side CCW until it stops.
I think I have about 10-12 wet sides that I rotate through so I can pull them out 2-6 at a time and swap in cleaned ones. Then I can soak and clean the others in citric acid when convenient.