Well, if the web site says 18" for the device + 2-3 inches for cables in the back...you should budget about 21" of space for the device. If you don't mind it protruding a bit in front (the feet aren't flush with the front of the amp), you can possibly get away with less.
I don't like "extreme" bends in my cabling so I always try to leave extra room in the back.
Cheers,
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