How can a 40 watt amp outshine a 140 watt amp

Don't forget, Gs5556 (catchy name, BTW!) that the amps under test are NOT run to clipping and are carefully level matched. In real world use, your objection is doubtless correct, but for purposes of this test? I'm less clear.

I think all the evidence needed is in front of us.

First, Carver pretty much proved amps sound different. He had to null his amp to the reference amp to make them indistinguishable. As far as I'm concerned that is 'game set and match' for the 'all amps sound alike' school.

Second, That null was valid ONLY for a particular speaker, though probably close on quite a few others.
The reason? Not only impedance but phase.
Try the same pair of 'nulled' amps on 3 speakers.... 1. A full range, single driver 2. Some Maggies 3. Some B&W from the '8' series.
I'll bet the null doesn't survive all speakers.

Now, keep in mind that the Clark test under discussion specifies NOT clipping the amp(s) under test. A 40 watt amp driving 83 db speakers with an impedance dip to 3 ohms at some wacky phase angle will almost surely clip. I'd be surprised if it didn't, if the level were above 'low'.

I would conclude that using a benign speaker load that it may very well BE impossible to distinguish 2 amps. Stereophile thought they could make such distinctions when challenging Carver. I'd be curious to know if Carver ALSO knew the speaker or had one of the test speakers with which to do his adjustments?

There are many subtle cues to telling gear under A/B tests apart. Even the best poker player can have a 'tell'. I'd suggest that the full test rules have a clue. Perhaps the exact level matching?

Easy: connect a 40 watt rated light bulb to each amp. Run a 50/60 hz sine wave at 2 to 3 volts rms into each amp. Eventually the 140 watt amp will blow the light bulb. There you have it. The 40 watt wins.

Thanks Gs5556 and Dpac996 for some very useful insight. I have a lot to learn, but it's going to be fun and I think AudiogoN will be THE place for my education.

"If you can't hear it, then no reason to invest in better amps!" by Grannyring.

Now there is a quote and a half! Let that guide you thru the Sea of Snake Oil.

I'll add that while navigating thru that sea, there may be differences that you won't hear the first tune but will, upon further listening, become apparent. If those differences are worth it to you then go for it. Vote with your wallet.

I just don't understand why it is that we all love this hobby if so much of it is truly unbeievable. As in "Snake Oil" etc. fixed delusions. I think we all believe that there are real differences in some aspects of audio or else we would join the silent majority of listeners content with whatever they use for sound reproduction.