I certainly wouldn't go through the headphone jack. The loss sn quality schrjnt be too high for me.
Please note: The unit cannot receive and transmit at the same time. The unit is either in receive mode or in transmit mode. I have tried to control the stereo system via the jack and then to send the signal to an external speaker via Bluetooth in my kitchen via the transmit mode on the RTX-1. In other words, I tried to listen to the same music in the living room and in the kitchen at the same time. However, I had a strong delay from the external speaker.
The RTX-1 has two connections: line-in -> the music from the mp3 player (jack), for example, into the RTX-1 cinch plug. So you need a one-in-two plug.
Line-out is from the RTX-1 to the stereo system. So per cinch plug (red/white).