I think you want to suck instead of blow (age old question of which is worse comes to mind...).
I believe the units are designed to use convection to let the hot air rise out of the top vents; I'd think it better to reinforce or accelerate that design principle rather than turn it upside-down.
Does your receiver get hot?
I have a Denon 1804 with only about 2" of space on top/sides, inside a glass-front cabinet. Even after playing movies at loud volumes, it isn't anything beyond "warm". I mean, I applaud your conscientiousness, but I just don't have the outlets or time to wire something else up, and I'm concerned that the potential noise is going to irritate me beyond the benefit. I'm also concerned about forcibly drawing dust through the receiver; the fans on my computer get much nastier looking than my hi-fi gear ever does.
