The sound is completely different. The HD650 are great and the mids are better. I however don't like the top smoky sound to the HD650. The top end is more clear on the X2. I hear a V sound on the X2. The mids are almost two recessed. The overall sound was more enjoyable on the X2 to me. I would own both but don't listen to headphones that much. Now I did think the HD700 was better than both of these. But way more expensive too.