I agree with the hot water. My grandmother used to use a flame and melted/distorted alot of lids to the point where the threads wouldn't work. I don't know if modern lids will still do that or not but I don't want to risk it.Hot water always works. i have tubes of 20 years .
Basically what I do, except I use a rag to expand the circumference of the cap.I set the tubes, cap down, into a sardine can with boiling water in it, enough to cover the lids. Sometimes I have to use 2 water baths, then carefully open with 2 pairs of pliers, one holding the shoulder of the tube, the other on the cap.