1 cm is nowhere near enough. especially for Onkyos which I believe have a reputation of running quite hot anyway (well the 605 certainly did anyway)
Your manual should give you guidance on clearance required. I'd probably have at least 10 times that (i.e. 10cm) as an absolute minimum for an amp.
(Edit not 10 times the manual recommendations but 10 times your suggestion, just reread my comments

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The PS3 also needs decent clearance on the the side/front. Top clearance isn't such an issue (the vents are on the side/front) but to enclosed a space can cause a build up of heat. The PS3 should warn you if getting too hot and shut down but if it is running warmer the fans will run faster and make it noisier.
Your other devices may be able to be accomodated with a smaller gap.