The new amplifiers is getting warmer and hot

Because the cpu for the video processor runs 100% unless you have it on economy mode. Most of the excess heat comes from the video processor. There will be a small patch on the top of the chassis that is much warmer, hot even, if you’re watching a long movie. That is precisely where the CPU is. I had this problem with my Denon X3400H a few years ago.

Expect the amp to get very hot by the end of a few hour long movie, and expect critical failure of many of these Denon amps (heavily used ones) from the mid-late 2010s to start failing in a few years.

Well they did have quadraphonic in the 70's
Have no idea what receiver or preamp was used
All I know is it didn't last long

Talk about comparing apples with oranges! What AVR from the 1970's are you thinking of?

I know a lot of people is using mini fan on AVR like Marantz Onkyo Denon etc.... I don’t think it was like that in 70s-80s and 90s 

@erik_squires  no I’m not comparing class D 
why the newest amplifiers ,AVR , AV processors run warmer than old school ones ?

The A 21 does run a few watts in Class A which will make it run warm

The Parasound A21 is A/B. I had the original, and they ran warm.  I measured the wattage, and as I recall, the idle power was 2x spec.

The other two I believe (but I am not Google) are class D.

But doesn't Class D run cooler? More likely that the heavier, more expensive to build heat-sinking capacity is reduced in the newer amps along with more heat tolerant components which allow them to run hotter without damage.

Are you comparing Class D amplifiers with Class A/B ? If so, that's your answer. :)