Good call there fezder, that looks usefull, esp if the line in takes priority without a bluetooth connection.
In my mechanical 'shop I have a dab table top radio, its has 6 preset buttons and 4 control buttons as well as a rotary encoder for tuning and a pot for volume.
It just so happens that the old wooden radio has 10 preset buttons with contacts, which are just right for the presets and control buttons, and I could rewire the volume control for mono on the radio board and use the old radios volume pot, and make a coupling that attaches the dab radio encoder to the back of the airspaced cap in the old timer.
The 2x16lcd on the dab can be replaced for a brighter one and go behind a darkened screen on the tuning dial so it hides when off.
And if the bluetooth module does default to line in that would automatically connect bluetooth to the amp when a connection to a device is made.
And maybe a class D raucous sounding amp for the speaker o/p.
Not too bothered about mp3 if the set has bluetooth.