Can I pair an iPhone6 running 9.3.5 with an iMac using 10.9.5?
It is not supported to pair an iPhone and a Mac via Bluetooth and use the either of one as a Bluetooth speaker/receiver. Both are in simple terms in the wrong device class

But you could use a third party tool to make your Mac an Airplay receiver. The one solution I found was AirServer. Start this tool on your Mac and it should show up on your iPhone as an Airplay destination.