Re: Omg ohms wtf!?
The designer of the JVM series has said you can mis match by one step either way and will be safe. That's just talking about JVM though
Mesa Boogie say the same thing with there amps. But apart from the JVM I don't know of any other Marshall where this is safe.
Bottom line. Every manufacturer is different. So just always match your impedence. It says that in the Amplifiers Manual so so in case of warranty you would be stupid not to.