20 Year Old Amplifiers compared to 2017

Thanks for this thread -- it's raising all kinds of points that make me realize how much I have to learn!  willemj started his post with a statement that I don't understand:
"Amplifier technology reached maturity in the seventies, when output transformers were abandoned in solid state amps. Ever since, well designed amplifiers used within their specifications have exceeded human hearing acuity. "

Could someone point me to a source that would help me understand this?  I thought all power amps, even Krells, use power transformers, so he must be saying that less sophisticated amps have two transformers, one of which is on the output stage.  Assuming this is correct, what is the difference in their function, and why is it bad for SS amps to have transformers on the output stage?

Sorry I'm such a newbie, but we all have to start somewhere.

@atmasphere,  thanks for your input —   That sounds like a plausible interpretation of what willemj  meant by his opening sentence. I’d be interested to hear Will weigh in here, especially if he meant something different.  As far as the second part of his statement (“exceeded human hearing acuity”), I would agree that is pretty subjective, as  evidenced by the many contrary opinions on this thread. 

Thanks for a great thread!  I am new to the world of higher end sound, but  wanted to thank the folks on this forum and Polk’s, for steering me toward a well used power amp. A year ago, I bought a pair of LSIM 707‘s, hoping to re-create the wonderful sound I heard on a friends high-end system when I was in my teens (in the 60’s).  Unfortunately, my home theater amp, a Denon AVR 1909, was not up to the task, so I put the speakers on ice until I could afford a better amp.   Earlier this year, I picked up a Parasound 2100 preamp, and as Christmas approached, I decided to see how much power amp I could get  in the ballpark of $1000.  I liked what I had heard about the Parasound A21,  but I  didn’t  have $2500 to spend on a new one, and I wasn’t sure I wanted to buy a used amp (even if I could’ve found a used A21 for close to a thousand dollars, which I couldn’t).  I decided to go with the A21’s junior sibling, the 2250.   Around that time, I started to hear  about the old McCormack DNA series on the Polk and Agon blogs.   I found a 20-year old DNA-1 Deluxe on sale for $1000, and decided to grab it and compare it to the new 2250.   Long story short:  The 2250 is a nice amp, but the old, dusty DNA-1 blew it away!  The new Parasound went back yesterday, and the McCormack will be staying (maybe someday I’ll pay for SMC or CJ to do the upgrade, but for now the stock Deluxe is all I need).  I’m a very happy camper, and am glad I listened to you all.  20-year-old amps are indeed worth the money, and more. If and when I  want to take my system to the next level, I suspect I will look into an even better pair of speakers.  But for now, I am just planning to enjoy a new level of audio for a while, rebuild my music library, and smile. Thanks, and have a Happy New Year!