Headroom is to protect your speakers. Under powering a
speaker is the easiest way to destroy it. The amp will most likely clip when trying to get the speakers to drive to an adequate volume and this will destroy the speakers.
Once you have the
headroom however, you can also damage speakers by over driving them but since the volume will be there, you should be able to hear this easier and correct it.
Also, because the amp is rated at 600 Watts @ 8 Ohms, you will not necessarily get 1200 Watts @ 4 Ohms. The
power to
impedance ratio is not directly proportional and doubling one will not always halve the other. This is why amps will often list the output for different loads and you may see 375W @ 8 Ohms and 600W @ 4 Ohms; or 800W @ 8 Ohms and 1200W @ 4 Ohms.
The best thing to do is look at the minimal load on an amp as this is the important factor. Thus if you have 1200 Watts at 4 Ohms as the minimal loading, it is a safe bet that you will have slightly more than half the available
power if you double the
speaker impedance.
Going below the minimal load value is going to damage your
amplifier.
You amp is putting our
power and expecting the
speaker to “absorb” a certain percentage of this, with a small amount returning to the amp. Once the
speaker impedance drops below the rated
level, the amp gets more
power back and will begin to overheat. It is important to remember also that a
speaker is not a resistive load. Its characteristics will change depending on what is happening to the audio signal.
The load only changes when speakers are added together on the same
channel. So if you have an 8
ohm speaker on
channel A and an 8
ohm speaker on
channel B, both channels have an 8
ohm load.
If you wired both 8
ohm speakers together in parallel, you would now have a 4
Ohm load and if you wired them together in series, you would have a 16
Ohm load.