Do my head and my cab have to match impedance ratings exactly? I have an 8 ohm head @125 watts running to a 4 ohm cab @125 watts. Does this mean that my cab isnt getting the necessary juice that it should be? (Aside from the obvious wattage deficiency of the head)

Also, my cab's rating is "4 - 8 ohms" according to the manufacturer's site. Whats that all about? Are they telling me that my cab can handle a max of 8 ohms and a min of 4 ohms?

And finally, I am considering purchasing a new head with considerably more power (400 watts) Is this safe to do considering my cab's specs? Will a 250 watt cab be able to handle 400 watts? (lets assume just for this example that both head and cab are 8 ohms)

Thanks guys
I need to clear up what an ohm is. An ohm isn't a measure of power, or volume. It's a measure of electrical impedance, or how much a resistor is taking out of the circuit. In this case the circuit is your head and your cab. Your head pumps out wattages right? But depending on how much resistance it finds in the circuit it will pump out varying amounts. If there is less resistance (4 ohm cab instead of 8 ohm) the head will be able to pump more power. The head doesn't have any ohmage, just varying amounts of wattage depending on the amount of ohms. This means right now, you're overpowering your cab if it is running at 4 ohms. What the 4-8 ohm thing makes me believe is that it can run at either 4 or 8 ohms, so to match your head and cab perfectly you'll want to run the cab at 8 ohms. If you don't you'll be overpowering it.