The Sony Ericsson Hi-Fi Bluetooth Headset with FM Radio
might work. It has a 3.5mm jack, so you can remove the headphones, insert a splitter, and connect one audio cable to your speakers, and one to your sub. I'm not sure whether or not you can permanently connect the bluetooth device to the charger, you may have to recharge it periodically.
I'm also not sure whether the iPad supports AirTunes. But, if it does, you could get an AirPort Express and send the music over wifi. Let us know if you found another solution.