Well, the head as well as the combo have a transformer support post.
Best to leave the 4 top bolts in place. Keep the tube shield in place to protect the power tubes and Rectifier tube which would be the one that would get damage if you slip.
Loosen the nut on the post. I use an open box wrench to loosen the nut, bring it up about half inch from the bottom. Then you need to use a smaller box wrench to drop the support. Once you get it loose, use your fingers to rotate it so it can drop down. 1/4 inch should be fine. Then you can remove the 4 support screws to remove the amp from the chassis. I had the same issue when I first tried to remove the chassis but then realized the support post on the transformer. If you fear you will damage the power tubes, you can remove them first, just do not install the rectifier tube in the wrong place or it will cause damage to the amp. Rectifier tube is closest to the power chord, check the tube chart on the amp plate or your manual for proper tube placement. Once you get the support loosened up, and removal of the 4 mounting screws, you will have to disconnect the reverb before you can remove the chassis completely out. The white wire is the hardest to remove, I usually carefully pry it off with a small flat blade screw driver. The cable with the molded plug is easy to remove. Remember orientation of the reverb connections. Molded plug is towards the front, and the uncovered RCA jack is towards the rear. It won't take long to realize you connected them backwards as it will squeal and sound terrible.
I normally start with the amp on the floor, front of amp facing down to loosen the mushroom support post. Then I rotate the amp so it is standing in the proper orientation to remove the 4 mounting screws. Remove the front two screws first then start with left side and remove the right side last. While removing the right screw, hold on to the amp chassis with the left hand. This is to prevent the chassis from dropping suddenly It will not drop very far as it will rest on the two side beams of the chassis. Removing the chassis, hold it by both transformers. Do not place your fingers into the amp chassis as the power supply capacitors may hold a high voltage charge. Also do not attempt to pull the chassis out after you had it operating since it will be very hot. Once you do the tube roll of the preamp tubes, let the amp cool down before attempting to reinstall the chassis. You may want to screw in the mushroom support slightly to get more clearance to install the amp chassis. When you get the amp part way into the shell, reconnect the reverb cables. Getting the amp back in is probably trickier than its removal but as long as you take your time, check for binding from the grill or flap of tolex around the opening of the front as this may impede installation. Installing the support bolts, start with the right side rear but do not screw down tight, keep it loose, next start with the right front scew, keep it loose. and do the left side, Always start the screws by hand to prevent cross threading. Once you get the mounting screws tight, you can adjust the mushroom support (do not over tighten this support as it will push against the transformer and bottom of the shell, you want it snug and then tighten the bolt. Once you get used to removal and installation you will find it takes less time than trying to get the tubes in and shields back while the chassis is still in its shell. I can remove tubes either way. If I am tuning for a specific change in power tubes, I usually remove the chassis. That way I can change tubes without having to wait for the power tubes to cool down. Best to turn off the amp when swapping preamp tubes. I usually keep the amp chassis out after tuning, and return a few hours later and recheck the tone (ear fatigue can result in well it sounds great now, but when you come back to it later does it sound the same?).