Sennheiser CX 6.0BT vs HD1 Free Bluetooth Wireless Headphones comparison. Sennheiser CX 6.0BT is 3 grams lighter than Sennheiser HD1 Free ! Shape of CX 6.0BT is like a barrel whereas HD1 Free shapes similar to a dumbbell ! CX 6.0BT has rectangular logo at the end of earbuds, as against round shape for the HD1 Free