I have a Qwest. Before it I had been wearing a couple of Shoei RF1000's - which were the quietest helmet I had ever worn (and much quieter than the Arai Quantum II I had). The Quest is noticeably quieter than the RF1000. And every review I've read say it's the quietest helmet they have ever worn.
Just to clarify, for a while I had the Quantum II and the RF1000. I rode my FJR at the time (which had a very low windshield on it) for a while with the RF1000. Then just for grins I put the Quantum II on and went for a ride. After about two miles I turned around and went home and put the Shoei back on. The Quantum II is the loudest helmet I've ever worn.
2015 BMW 1600 GT
2016 BMW S1000R
2012 Ducati 1199 Panigale
Previously owned a 2005 and 2009 Goldwing