Just received my 650 last weekend. On my first trip with it, I noticed a hidden benefit. On flights I like to listen to music using Bose QC-2 noise cancelling headphones. On the 600 I had use use the low sensitivity setting, and turn the volume up to almost maximum, with resulting reduction in quality.
Now with the 650 I found that I can use the high sensitivity setting, and have to turn the volume way, way down! Quality is excellent.