It’s worth noting that the stock tips have two notches that effectively increase the nozzle’s length (for better fit and potential bass quality). Another third party tip that allows the “2-mode” configuration is the spinfit one (because of its tight fit).
I find best results when the ie900 sits right at the ear, meaning I avoid pushing it deep into the ear canal. The result is a much tighter bass response I’m guessing because of the increased distance. It could also be the spinfit tips since those have a slightly narrower nozzle than stock.