See the 2016 review article posted by MDB in 2017. Also towards the end of that thread, I reference a 2006 Widex technical paper cited in the review on open vs. closed fit. Both articles say that you don’t get maximum benefit out of noise reduction and directionality with an open fit. And also imply that you may get better speech recognition with a more closed fit because of potential cancelling interference between sound from your HA’s and possible out-of-phase sound by passing your domes, depending on the acoustics of your ear canal. I like a more closed fit as I perceive the above to hold for me but maybe part of its just the placebo effect! (for me).
P.S. Just checked. The direct link to the Widex Pro site for their 2006 technical paper is back to working.