There are plenty good receivers and amps with iPod 30pin dock. Of course there are Bluetooth adapters cheaply available (per-amplified or not). 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.

See: ipod-30pin_dock

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.