I'm guessing the manufacturers suggestion is probably a bit of overkill? In this situation I don't have a lot of choice. It's either an inch on top or no stereo in the living room.
I am a guitar player and have used tube amps for decades. Many guitar tube amps are completely sealed with no ventilation at all and most of the rest have very poor ventilation and yet these amps can be used hard year after year or decade after decade. I've done many gigs outside in the heat of summer with only favorable results. Are stereo amps more susceptible to heat? How many tube preamps actually get 6 inches of space above them in the racks most of us use? What would be the temperature difference in 1-6 inches of clearance on the top?
I'm not arguing just trying to get to the bottom of this because it is important to me. I do appreciate all of the replies.