All HyClean/AirClean bags should have a silicone gasket on the bag collar. If you still have your HyClean bags, would you mind taking a picture and posting it here?
As for the meltblown polypropelene generics available here in North America (DVC, Envirocare Technologies and 3M/Arm & Hammer), unfortunately none will filter as well as the original Miele bags. However, if I absolutely had to use generic, my choice would be the 3M/Arm & Hammer.