Difference between amps

Real life performance differences of multi channel amps depends on your application and supporting gear. Most of the modern multi channel amps are built for HT duty. They do a nice job of powering the sound of the chop of a helicopter blade or the impact of an explosion. The power ratings in 2 channels driven versus all 7 may be because in normal HT use, rarely, or only in very brief moments are all 7 channels be driven equally. Usually only the center channel is driven continuously. So, in their mind, continuous RMS across all channels is less important? Ultimately it comes down to how it sounds in your system. Specs can’t tell you that. But do not assume these types of amps can sound as good for 2 CH music as a dedicated 2CH amp designed specifically for music reproduction. Different applications, different design goals. So if music is at least one of your priorities, get a dedicated amp for your mains, maybe a decent integrated amp with HT bypass. This should help maximize your music experience. And then get whatever you like to power the rest for HT (or other) duty. If you are comparing between multi channels amps, you need to level match the volume from one to another. When you A/B them correctly, differences in HT applications can be less significant than you might expect.

"...In 1968 Sansui was in the top high end and in the mass market of top high end..."

Not sure if that was ever true.  The  Sansui AU 7700 could never drive the hi-end loudspeakers of the day like the hi-end Infinities. 

I think we rely too much on power ratings.

It takes 10x the power to double the volume, so do I care a lot between 120W and 100? No, really I don’t. In most homes, with a sub, and speakers run as satellites (not full range) 100W per channel is way more than enough.

Honestly, if you are running satellites, with subs, 75W amps per channel x 5 is more than enough unless you are running a mini theater in your home.

How the amps work with your speakers and how they sound is a much bigger deal. I like NAD for the use of nCore, and Parasound a lot.

They are notorious for misleading the consumer with their power output ratings. But that's a marketing decision.

Correct. All this stuff is. The biggest marketing decision is the one virtually all of them have agreed on, to confuse and mislead with meaningless numbers. All these numbers you're so concerned with are meaningless. Absolutely meaningless. 

What are the real or should I say physical or performance differences among these? 

Drop real, drop physical, whatever these even mean, and stick with performance. Then realize the one and only performance parameter with any meaning at all is how people say they sound. Because that is all that matters.  

It will take you a very, very long time listening to a lot of different stuff to realize this, but amplifier specs simply do not matter. Only one spec matters: speaker sensitivity. Good luck if you buy speakers with less than 92dB sensitivity. Good luck finding an amp. When you read all the problems people have matching speakers and amps first thing you should do is go look up the sensitivity of the speakers they have. Go. Look. You will see. Happens all the time. Guy just the other day took a pass on some very excellent easy to drive speakers, bought some 89dB when could have had 99dB, the ones he got will require TEN TIMES the amplifier power to play the same level.   

Good luck with that. Really superb sounding 50 watt amps are all over the place, and are actually affordable. He needs 500 watts. All because he screwed up the one thing that matter about amps, and that is speaker sensitivity. Good luck. 

So forget amplifier power. Do this one simple thing right, find speakers that sound good and are at least 92dB sensitivity you will have no problem whatsoever driving them with anything from 20 watts on up. So power ratings, forget em. Why I said they are meaningless.  

Or you can obsess with the marketing malarkey in which case, good luck! 

Other thing is, multichannel. Sorry, you want 2 channel. Stereo. Multichannel is a wasteland of crap sound quality. They suck people in with the HT BS that you need umpteen channels to watch a movie. Not true. Not even close. https://systems.audiogon.com/systems/8367 Movies on this stereo system are enthralling. Music is to die for. Posted for a reason. Study and learn.

I wouldn't know. I only buy mono amps. They are the only ones that can be installed correctly.