How can a 40 watt amp outshine a 140 watt amp

A 40 watt amp can out perform a 140 watt amp in two ways. First, a watt is current into a resistance, and the resistance is the speaker. But the speaker resistance varies with frequency. As the resistance offered by the speaker drops, the amp has to put out more current. If the 40 watt rating is based on 8-ohms, the amp would have to put out 80 watts into 4-ohms and 160 watts into 2-ohms. Only if the 40 watt amp has a capable power supply can that happen. That requires an expensive power supply (bigger transformer, higher quality and capacity of filtering caps). The Pioneer 140 watt may not have that kind of a power supply. It is rated at 140 watts at 8-ohms, but if the speaker load drops to 4-ohms, the amp may not put out 280 watts to drive the load, instead the power supply only puts out, say 200 watts, and at 2-ohms it says "I give up" and just clips.

The second factor is the quality of the output devices. The more expensive transistors are more linear and stable and distort the signal to a far lesser degree. High quality output devices in a smaller watt amp will definitely out perform a mass market higher powered amp.

When comparing amps, it is critical to note the rating at 4-ohms and, if able, 2-ohms in addition to the standard 8-ohms. The ability of an amp to put out current to a varying load maintains its sound quality throughout the audio band. But that is what drives up the price of an amp -- the power supply and output devices.

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.