I have a Garmin Zumo 660 paired with my Sena 20S. My iPhone 6S is paired with the Zumo so I can use the GPS to visually screen calls and dial from the screen. I also have a SM10 to route music from an iPod and audio from a radar detector to the 20S. The attached PDF illustrates my setup. I wear ear plugs while riding, so I have the 20S turned up fairly high. The system all works (sort of), as I can make/receive calls, hear the radar detector and iPod music and audio from the GPS, but the GPS audio link to the 20S seems to remain at maximum and therefore is highly distorted. However, when I get nav instructions from the GPS, it's at max volume and heavily distorted. The audio from the GPS to the 20S is essentially uncontrollable. There's little/no difference in the volume in my headset whether I have the Zumo 660 GPS audio set at its minimum level (10%) or near maximum (80%). If I quickly turn the Sena 20S volume down to an acceptable level for the GPS, the music from the iPod and any radar detector alerts (via SM10) are inaudible. Music played from the Zumo 660 is also heavily distorted, but if I hardwire it to the SM10, it sounds great.