I've just installed my Yamaha RX-V1 in a rack. I'm concerned about it overheating. How can I determine if the unit is receiving proper ventilation? The rack is pretty much full so I don't have a lot of slack space. Currently there is only an inch of so of space above the unit. Are there any rules of thumb to follow here or is there some way I can monitor the temp with a gauge etc? If so how warm is acceptable?