Thank you all for your input, reassurance and optimism. I do feel more confident in the unit now. 👍
I saw those holes - somehow I am not that impressed with them.
If it were my unit then I would let it run for some hours in its working position and then measure the cabinet temperature. It will be warmer inside 🙂
In fact @rayma said "not nothing", in other words: it is "something" (to take into account).
Don'take me wrong, an oversight may happen to everyone. 🙂
It certainly will be, but it was designed that way since it's in pure Class A, and it's a Yamaha, and it wouldn't have lasted to this day (at least 15 years, I guess) if it had been designed badly.
Of course, it may have failed, but the experts (not me! 😉) don't think so.