If the unit was as you say when it was put away you should have no real issues. I would have someone qualified take the lid off and blow out the dust and perhaps clean the contacts on the boards. Re flow the solder joints on the power supply transformers and you should be good to go. One thing I would document is the condition of the tubes before you fire it up though. Closely check to make sure they are free from wear or types of burn.
Mike
Mike