Theritically 650 easier to drive than 600 Coz the impedance curve is more favourable. See headphone. For graphs. The impedance published is at 1khz but the most current needed is at the bass end and the 600 is double the impedance at 100hz whereas its not for 650. If you read too much the net will tell you that 650 has a veil but apparently the new ones have new drivers which solve this problem ...