I put a stainless steel mug on top of the case and that did the trick as a 'heat sink'. It was actually getting hot as it's 35 degrees C here at the moment. Why is it getting hot with nothing playing?dx3 pro+ only consumes 4W and with that enclosure it's never a problem.
You could install a small 12x8mm heat sink on top of the hp amp buffer, or even better, a 4.5mm thick thermal pad under the pcb.
But it won't offer you any real benefit as there's no heat issues in the first place.
just relax and enjoy your music.