After trying these tubes I don't understand why they're not more popular, or why they don't seem to be available from any US or Canadian distributors anymore.
All issues with supposed "hum" are nonexistent, at least in my Mark IV. I've even heard Marshall used these tubes at one time, (I'm not sure about currently.)
Also an interesting note, since I purchased mine from Germany they still have "Svetlana" written on the tube.
cesjr