How can I connect my iPhone 6s Plus to my stereo?
I've done it 4 different ways (from lowest to highest cost). Sound quality is mostly dependent on the quality of the audio file on the iPhone and the quality of the stereo. The quality degradation due to any of these 4 connection mechanisms seems relatively low - unless you're an audiophile or Neill Young. ;-)


Connect iPhone headphone output to stereo auxiliary input with an appropriate adapter cable (mini-stereo jack ==> what ever your stereo accepts e.g. RCA). Important: Run iPhone output volume only around 50% to avoid input distortion at stereo. 
Get bluetooth receiving device (I've had good luck with one from Philips bad luck with some generic chinese stuff), which connects to the stereo input. Then connect with bluetooth to that device. In my experience bluetooth worked rather well when the iPhone was stationary reasonably close to the bluetooth device. Moving around with iPhone in pocket while on Bluetooth connection could cause audio dropout grief in some cases. 
Use an Airport Express as wifi or wired network participant (e.g. as a wifi extender) on your regular home wifi, and connect the audio output from that to the stereo input. Then use Airplay to send the iPhone audio to the Airport Express. Moving around with iPhone in pocket seems to be more tolerant than bluetooth based connection. 
Use an Apple TV as wired or wifi participant on your home network. However to the best of my knowledge the Apple TV only outputs digital audio (HDMI or optical SPDIF on 2nd or 3rd gen Apple TV), so either the stereo has to have inputs for HDMI or optical SPDIF or one needs an additional converter box.  And then use Airplay to send the iPhone audio to the Apple TV. Moving around with iPhone in pocket is equally tolerant as with Airport Express, since both use iPhone wifi.