The Yamaha A-500 does NOT have a SPDIF Input, therefore you will not be able to use your Yamaha A-500 to monitor from the SPDIF Out of the FS Project. We do not recommend using a home stereo for a studio monitor system. The RCA Line Output is Red and White indicating Analog audio.
SPDIF carries 2 digital signals for Left and Right over a single RCA Type cable that requires use of a single unbalanced SPDIF cable (75 Ohms). You can get these at your pro-music store. There are 2 jacks on the back of the FireStudio Project. SPDIF IN and SPDIF OUT. Each Jack represents 2 channels.
Another poster commented and is correct that you would need to acquire an after market Digital to Analog convertor that connects SPDIF on one end and Analog RCA Out on the other to connect to your Yamaha A-500 Receiver.
This is not true for all receivers, as many receivers do have SPDIF Input, however in many cases, these inputs are Optical and not RCA / Coax connections. In the event someone else reads this, you can get a format converter to convert RCA SPDIF to Optical. These are available online, pro music and broadcast stores, and some higher end home theater stores.