I have had two friends do the exact same thing with Mesa boogie amps. Whatever you replace the old cabs with will be better than OEM. The gorilla glue will be great just put water on the wood before hand, the more water the better gorilla glue works. For the joinery, the finger joints will probably double the surface area for glue. Great idea with biscuits but I would use a spline strip the whole length of the joints as the materials are so different from each other, but that’s just me, I have done those allot with no biscuit jointer available. Also their is no shame in using metal fastening “L” brackets inside to strengthen the corners, you are dealing with a lot of weight and vibration.

CHRIS, Charlottetown PEI Canada. Anytime you can repurpose, reuse, or recycle, everyone wins!