McIntosh MI-75. Any info?

Can anyone please supply me with information on an McIntosh MI-75 mono tube amplifier? There isnt much info on it on the web other than that its an industrial amp and that its rather rare. Is it a good unit to pick up? Does anyone know what they usually go for? I found someone locally who wants to sell one but I havent the slighest idea what its worth. Thanks for any help you can give!