Very nice design 
. I’m curious about the final result. And I can assure you: this boombox will be heavy 
. My smaller boombox (40x25x18cm, made of 15mm plywood) weighs about 9kg.
I think it’s a good idea to use the 2.1 plate amp instead of the standard 2.1 amp. You will have plenty of space on the back side for it and will not need to worry about where to place the controls and connectors. This is always a headache when using the 2.1 amp. However, I would try to place at least the status LED and the IR receiver in the front panel (maybe somewhat hidden) and use the remote control that Arylic offers or the smartphone to control the boombox.
Since I have a 2.1 plate amp lying around in search for a project, I might steal your design when you’re finished 
.
Have a nice weekend.
Sven