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No one in this sub-forum actually plays the guitar so I doubt they would ever have the need to replace cabinet speakers or work on them.
But because I'm a nice guy, I will tell you that there are some options. I had the same problem with my Avatar 2x12. Place a block of ice on the back panel at one corner and then heat up said corner of cabinet with a hair dryer. Then, get a putty knife, butter knife or flat head screw driver and pry that bitch open. OK, just kidding, that is a last resort. What I did was a "reverse punch". If the wiring plate is in the center, as it should be, you can try taking that off and then 'reverse punch' the back panel off. Also, I've heard of people getting a small jack of sorts and jack it off.