Difference between amps


OK so let's talk amps.  I'm sure this has been addressed before but are there real differences between  multi-channel amps among the various brands in the mid-fi to high end markets with comparable specifications? Adcom, Anthem, Classe, Emotiva, Krell, McIntosh, Marantz, NAD, Parasound, Outlaw, Rotel, etc.  What are the differences in these brands?   I can site one difference.  In multi-channel Marantz amps, the specs never state "all channels driven".  They are notorius for misleading the consumer with their power output ratings.  But that's a marketing decision. What are the real or should I say physical or performance differences among these?  .  
pdn
"...Only one spec matters: speaker sensitivity..."

I know you have posted this over and over but why limit yourself? Sure efficient loudspeakers make it easy to buy amplifiers, but that eliminates a lot of VERY good top end loudspeakers. I'm not willing to do that but I'm willing to buy an arc welder of an amp if I need to. .   
@redwoodaudio  Check out the impedance of your speakers.

https://www.stereophile.com/content/harbeth-monitor-302-40th-anniversary-edition-loudspeaker-measurements

If you averaged it over the entire spectrum it would be well over 8 ohms.  Sensitivity is not the only measurement that matters.
@twoleftears that's my point... easy to drive, yet sensitivity under 90 dB (87 per stereophile measurements as you linked).  limiting one's self to "highly efficient" speakers over 90 dB sensitivity, without regard for other factors, would be senseless except when using flea-watt amps.
Thanks everyone for an excellent response and participation.  This has so far been a very active thread with very meaningful feedback.  I've learned a ton with many takeaways.  Although I do have a 7.2 surround sound system for movies, etc, music is still my first priority.  I listen mainly in multi-channel stereo.  One thing I learned here (and others have implied this to me but I forgot about it) is to use a separate higher output two channel amp for my mains and then a 5 channel for the surrounds and center.  I'm going to make this change right away.  Another thing is that for the $$, it seems Parasound is a super choice.  I may sell my Marantz MM7025 and purchase a Parasound two channel amp.    THANKS again.
I ended up with an ARCAM AVR 550.  However, I wish I could have afforded to pay $5,800 for the ARCAM AVR 850.  Their G technology is incredible, as the first 50 watts are pure A.  I had a real problem with their customer support and they are probably the worst in the business.  He was a real jerk and hid behind his phone.  Not sure what I would buy less because I want to be able to have an amplifier that can handle both 2 channel and home theater.  When the time comes I will call upon this group to help direct me.