I am using AD815 in 1MHz for 75ohm load in differential mode as depicted below ( my buffer gain is 1.6v/v). I get good output swing of 40vpp. Beyond that output level, I get bad output results ( collapsed non sine signal). Even I see similar bad behavior for output below 40vpp. I had strong spice results of 60vpp output for this IC. Please note once amp wants to give bad output results, it starts to gradually draw lots of current from power supply.