I had the same problem!
Here's what Bill from Morningstar recommended...And, it worked...
"The tube socket has a death grip hold on those tube pins to say the least. You really have to wiggle(Back and forth) ever so slightly while pulling at the same time. You just have to work it out slowly.
I find it a bit easier to take the top cover off as my fingers seem to get a better grip this way.
Ridiculous as to how hard it is to get the tube out but the tube socket is a very good one."
Here's what Bill from Morningstar recommended...And, it worked...
"The tube socket has a death grip hold on those tube pins to say the least. You really have to wiggle(Back and forth) ever so slightly while pulling at the same time. You just have to work it out slowly.
I find it a bit easier to take the top cover off as my fingers seem to get a better grip this way.
Ridiculous as to how hard it is to get the tube out but the tube socket is a very good one."