Just to add to my answer, I am very new to HA and decided to try some NHS ones, these were fitted with a double dome unvented. I very rapidly decided to go down the self programming route and bought some ebay Siemens Binax 7 and stated with double domes again.
Then I started experimenting, because I could and tried some vented domes, liked the sound and lack of suction a lot but could not get the gain to where I wanted it.
Then upgraded to some Siemens NX Charge & Go, great not having to change batteries and tried some Vented Sleeves which I am running with now. Extremely pleased with these, no suction, can get the gain as high as I could with the double Unvented domes. They are by far the most comfortable, stay put in my ears and I never find myself checking them.
So perhaps they just happen to suit me but worth a try. Only small downside is that the small vent holes can get waxed, so I fully clean each night, but a small price to pay for the comfort & performance I get.