amp head/cab ohm load

Which head?

Around 300 or so.

 

Each amp is different, so there is no generic answer. @Stumbo 's response is a good estimate though.

 

Less than 300 watts, but so? The potential volume difference between 500 and 300 watts, assuming all else is held constant, is much less than you might think. To be twice as loud as 250 watts, you need 2500 watts converted to "loudness."

 

What does the user manual say?

8 ohms is about 50% of the 4 ohm figure.

 

There is no safety in numbers. Use your ears. Wear earplugs. Keep turning up until it does not get louder. At that point you are giving your speaker(s) death. Turn down and never turn up that far again.

By reducing the amount of lows in your sound you can turn up louder. It may be that they get to sounding quieter. Turn off before you smell them dying.