/diy/ - Do It Yourself

>>1216233
The tuning knob is probably connected to a big mechanical capacitor bank inside the case. Sight unseen, the best bet may be to clear all that out and replace it with a 6-position rotary switch such as Pic related, with a matching shaft type and length.
Of the three devices, the rasPi will have better I/O capability for the extra controls. You can either poll them in a little app program that watches the GPIOs and sends commands to mpd or kills and restarts mplayer or whatever, or if you feel bold you could try your hand at device tree overlays and use gpio_keys and triggerhappy to run shell commands.