Powering up a SET amp after 6 months in storage

What precautions do I have to use in turning on my SET amp after it has been sitting unused for a number of months? I have heard the phrase "bring the amp back up slowly" in connection with tube amps and preamps, but I don't know exactly what that means, and if a tech guy has to do this for me. FYI, the amp has a mute switch in the back if that is any help. Thanks.