Preamp tubes specially designed to prevent microphonic problems. I just installed them in my Mesa Boogie Triple Rectifier and they are very quiet indeed. I'm very satisfied.
As far as I know, these are the only 12AX7 preamp tubes available on the market with a magnetic sheet wrapped around them, but I could be mistaken. It is there to prevent, as much as possible, microphonic squeals and other nasty noises.
These preamp tubes are a little more expensive than the regular 12AX7s but, of course, one pays a little extra for a better product.
So, overall, great quality preamp tubes and the people at Thömann are super fast with the shipping and take great care in packaging the items!