The amp is rated at full on power... I have never ran an amp on 10... mostly because it usually begins to distort. I'm a believer in head room. Most amps I've ever owned I rarely got past 5. Keep in mind... 5 on your master volume is more than half the watts available in most cases. But if your head is 600watts I'd be surprised if it put more than 400 watts on half volume. Besides an 8ohm 400watt speaker will handle a 600 watt amp at 4 ohms. Ohms is the resistance of the speaker, meaning it will be like applying the breaks to your car... it won't let the amp hit it with all 600 watts. You get into trouble when you go the other way. If your cabs were 400 watts at 2 ohms your amp would be like a truck rolling down hill with no breaks... it's gonna crash. It would actually hit your speaker with about 1000 watts and you'd probably only be able to get away with playing with the master volume at about 3.
Be careful when adding cabs... When you add cabs you aren't endangering the speaker... but you are endangering the amp. If you add too many speakers you can lower the impedance below the amp rating. In doing so your amp can draw too much electricity (trying to feed the speakers with voltage) and basically just fry your amp.
Here's a link that may help explain what's going on.
http://www.prestonelectronics.com/audio/Impedance.htm
