4 ohm and take care of them.
+1 to vertical stacking. Using 4 of them at once is amazing.
The 4 ohm B2's here are run on QSC 2402's(700 watts available to ONE cabinet). I had asked Andy at ACME about it and he said be very careful since it's like having a 700 HP engine in a street car. Nice and fun if you're careful but wrapped around a tree if you're not.
A dbx 166XL is used before the amp with the compression set to be almost inaudible and the limiter on. Even with that, an eye is kept on the speakers(cone movement).
If you want to find truth, start by turning off your television.