If you have a Rotozip or a rotary too. (Dremel, etc), I would cut a groove near the damaged side in pictures one and two. (On the side, not the top or bottom)
If you think it will be cutting it too close to the chassis, then cut a groove as far in as possible and use a chisel to open the rest until you have a groove cut through the wood side.
I would then try and spread the cabinet apart far enough to slide the chassis out. you could use a piece of wood or something to hold the cabinet apart while you slide the chassis out.
You might be able to repair the damage to the cabinet then and save some bucks on a new one.
I actually dropped it (on purpose) from a 2 ft ledge in my garage and it landed perfectly to break the small inner wood blocks that support the chassis when you slide it in there.
short story: alls good now, the amp works fine and I screwed one screw in the top back from support.
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