I've noticed that my older Burson integrated, with its two 500 watt toroids never got 'hot' no matter how long I left it on. And that was with a run of the mill (but better than stock) power cord from GTT Audio.
The newer Burson, with 'only' a 380 watt toroid and a Zu Mission PC gets very warm to kinda hot if left on for more than 4-5 hours. Having no venting and using the entire casing as a heat sink would lead one to conclude that this is why but the older model was designed that way as well.
Tis a puzzlement!