Denon/Marantz's "stable on all channels" claim should be legit because they covered themselves by including the following instructions in the Owner's Manual.
View attachment 29510
If he has been doing it and have had no problem, then it means he's a sensible person and/or his speakers/room, despite being 4 ohms, do not need a lot of power. Remember, as shown in the often linked calculator below:
https://myhometheater.homestead.com/splcalculator.html
Power requirements do not depend on one single factor, it depends largely on the required sound pressure level (spl),but also depends on:
- seating distance.
- speaker's sensitivity in X dB @2.83V/1m or @1W/1m
- speaker's impedance
- room gain (near wall, corner etc.)
Required SPL is obviously the key factor. For example, if he watches movies at 75 dB average, 95 dB peak, speaker sensitivity 90 dB/2.83V/1m, that is, 87 dB if impedance is 4 ohms, and his seating distance is 10 ft, then the calculator would show he needs 30 WPC into 4 ohms. The A1050 obviously would have no problem doing the job. If however, he needs 85 dB average, then he will need an external amp that can do 300 WPC into 4 ohms.
Regardless, I would definitely suggest he put a fan or 2 on top. The build in fans should be adequate to protect the unit from dying within the warranty period (my educated guess) for normal use, but adding the extra fans on top will likely extend life well beyond the warranty period.