I am hoping you have been able to remove the tube by now. Here is the simplest trick that I have found effective on more than one occasion.

It is natural to hold the barrel and apply force to the choke tube wrench. Instead of doint that, put the choke the tube wrench in a a vise, then push the barrel against the wrench and turn the barrel to loosen the choke tube. Far more force can be applied via leverage this way.