Because of the open paradigm on the OPN by design, you’ll never be able to get rid of sounds around and behind you so that you only hear the sound in front by making fine tuning on the gain controls. That’s why as soon as the audi starts fine tuning in the gain control, you’ll start losing out on what you can hear, because you’re not just cutting gain behind and to your sides, but you’re cutting gain all around you, including in the front.
It is just like that by design, it’s not because it’s lacking noise reduction. It actually does (but very subtle) noise reduction on surrounding sounds (they call it sound “rebalancing”), nevertheless, its main focus of noise reduction is by cleaning up the speech clarity in front from the noise, but not really by removing surrounding sounds.
I would suggest to top out on the Max Noise Reduction setting for both Simple and Complex environments in the OpenSound Navigator screen, and just start letting your brain to get used to the surrounding noise so that eventually the brain hearing will do the tuning out of the surrounding sounds and focusing in on the speech in front. If this is your expectation, and you don’t keep trying to fight it by making your audi turn down the gain controls, you’ll have much success with the OPN. Otherwise, your audi will never really be able to meet your expection with the OPN of trying to block sounds from around and let sound in front through. That’s just not how the OPN is designed to function.
If you really dislike the open paradigm, you’ll probably be better off with the Marvel.