There is not a single component in electronic gear that does not have a set of specifications on useful life that does not show decreasing life as a function of increasing temperature. I personally would not gamble with running hot.
At a local shop which only sells tube electronics, I talk a lot to their repair person and MUCH of their service repairs involve overheating, including some wild events like components falling off of the circuit board because it got so hot that the solder melted. In almost all of the overheating cases, someone put the component in an enclosed space that did not allow for sufficient ventilation.
If you must put it in a place without sufficient clearance on top, you should do as someone suggested above--run at least one computer muffin fan to move the air around the component. Regardless of the ventilation condition, it is NOT a good idea to leave tube electronics on all of the time--turn it on when one wants to listen; tube gear sounds decent after only a few minutes of warmup.
