I think you can go without the empty spaces as long as the units above, and especially if the one below…don't run real hot.
Usually the side of the unit that has the tranny, and or tubes...is the hottest.
I make sure I don't have units where the trannies all line up one over the other when I stack 'em in the racks.
My racks are all pretty full too...if I left all the spaces above & below for ventilation...I'd need another set of racks. 
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miroslav -
miroslavmusic.com"Just because it happened to you, it doesn't mean it's important."
