Question about Yamaha A-S501

First check whether your DeCon speaker has a line-in (3.5 mm jack). If so, you can connect it to the record-out of the "Yamaha A-S501" via a "2xChinch - 3.5 Jack" cable. This way you can transfer music from the Yamaha receiver to the Geneva DeCon.
A possible cable would be this one: Primewire Audio Cable / 2x RCA to 3.5mm (1 m, Medium, RCA, 3.5mm Jack (AUX))

To connect the whole thing the other way round, i.e. to transfer music from the DeCon speaker to the Yamaha receiver, I only see the possibility of working with a Bluetooth receiver.
A possible model that can be connected to the CD-In of the receiver via Chinch would be the following: Logitech Bluetooth Audio Receiver (Receiver)

The connections mentioned should work at least in theory, but they do not necessarily support the audio quality.