I am running a Yamaha bbt 500h head to my 2x10 (4ohm) cab. The head gives 250watt at 4ohms, and 500 watt at 2 ohms. But will it give if I add another 8 ohms cab? I heard one 4 ohm cab and 8 ohm cab will result in 2.67 ohm. Will my head still only go 250watt? Or will it go 333.75 watt? Is it bad to mix different cabs with different impedances?