This could be attributed to the fully open design on the 660 verses the open port on the GSP500. I will be wearing these a lot this season. It was after a session of rows, though, when my grip was fried way before my back muscles were tired, that I took heed and bought some new straps. Im over the moon with them. They are well made and comfortable to a certain extent, however, buy a size up. Highly recommend as everyday, stick em in your pocket headphones that you will enjoy using & at a price that you wont panic about them getting lost or damaged. SOUND - mind blowing for this price, mids, highs, bass, you can hear and feel every note, instrument and voice! Awesome The case is everything described and more! Why is so difficult to find this product with velcro flaps?