See if you can find a can of Kroil oil.That is some of the best penetrating oil on the market.soak the threads really good and try again.If you have a wire welder you might be careful enough to dab a couple of little goobers on the inside of the tube .Then maybe you can use some kind of needle nose pliers and open them enough to try to un screw the tube from the inside.Be sure and check with the other guy's on here and make sure I'm not sending you off in the wrong direction.