There are plenty good receivers and amps with iPod 30pin dock. Of course there are Bluetooth adapters cheaply available, some amps need pe-amplified models. I have not come upon a version that properly supports remote control commands, so let's create our own. Then a replacement PCB for Yamaha R-840 can be created easily.

For modification of iPod Dock on Yamaha R840 (with remote control) see: Yamaha iPod 30pin dock
If your amp has USB iPod input, see USB iAP section of Yamaha page.

Some older players (e.g. car radios) only provide 12V via firewire lines (pins 19+20) instead of 5V (pin 23). Use an LDO or switching regulator to get desired voltage. Adapt voltage regulator for BMS to your needs.

#TODO insert iPod links

https://pinoutguide.com/PortableDevices/ipod_pinout.shtml
AdrianGame : iPod Accessory Serial Protocol
https://web.archive.org/web/20080622202730/http://stud3.tuwien.ac.at/~e0026607/ipod_remote/ipod_ap.html
http://www.ipodlinux.org/Apple_Accessory_Protocol/
xtensa/PodEmu: Android application that emulates iPod to use together with old 30-pins iPod serial interface
PodEmu on XDA
xtensa/oap: Implementation of iPod serial communication protocol used in old 30-pin iPod interfaces
oandrew/ipod-gadget: iPod usb gadget for audio playback

#TODO insert communication protocol