The PIS Advanced has an adjustable first stage and a single strength second stage.
I have a PIS Advanced and mine is fully adjustable. Hmm...
When I turn it on, I turn it to the middle of the dial and keep it there for the "let down" and the regular pumping.
Try turning the knob during the second stage and see if there's any difference.
When I first started pumping, I had it almost at the lightest setting so i could barely feel it, but it helped "toughen" my nipples. Just 2 days ago I turned it up and was happy to not feel pain AND to be rewarded with additional milk!
Make sure you're using it correctly and call Medela if you still have problems.