Please help me understand

I do not think it is true that the human ear is more sensitive than any measuring tool. But what is often claimed is that while some levels of distortion are measurable, it is provably inaudible.

My argument would not be that such a claimed level of distortion is in fact audible, it would be that we are not appropriately measuring distortion through the entire system, and that the THD specs often listed do not accurately represent real world system wide distortion, not least of which being the interaction with a real world speaker.

Two cars may have an identical 0-60 mph number in a straight line, but once you get on the track, things like wind resistance, weight distribution, front versus rear versus all wheel drive, gear ratios, coefficient of friction, etc all come into play.

THD as a standard measurement does an ostensibly admirable job of presenting a metric given a very simple and constrained model. But it does not account for the bends on the road.

If i take a vision test and have 20-20 vision does that mean I can see colors?

"If i take a vision test and have 20-20 vision does that mean I can see colors?"

no, but you could take this test and check if you do:
http://www.opticien-lentilles.com/daltonien_beta/new_test_daltonien.php

each manufacture when designing amps/speakers/preamps etc - are largely basing their testing on measurments and actual listening - now, depends on the equipment they have available
(the rest of) - it might sound perfect with what they have.
But it does not mean that it will sound same with other combination (for example Accuphase preamp with McIntosh power amp and reverse - just does not sound as good as manufacture match - if manufacture match, then sound is good on both but different). Simply speaking - each equipment has some kind of "equalization/sound proceesing" circuitry that will effect actual output. After that is up to the buyer to decide if they like it or not...
There are sound correction device (like McIntosh MEN220) that tries to compensate for all the equipment/room and make the output more listener "friendly" - but in some cases it can not.
As mentioned before about Bob Carver - yes, it is possible to make 2 different amps sound exactly same using different components - but why? if it was the case, then we would not need all the audiophile forums and discussions.....
so trying to achieve perfect sound by swapping components - it is a endless job, every time we change some component, we gain somewhere and loose somewhere else - but all this keeps audiophiles busy and generates nice profits for manufactures

Rok2id,
Who`s discounting science?it can explain alot but not 'every thing'. Certainly we`re able to detect audible characteristics with our ears that can`t be verified by measurements(yet), Why is that so hard to except. We can measure many things we could`nt 30 years ago and I`m sure in the future we`ll measure things we can`t today. In the mean while we have ears to rely on. Rok2id I`ll assume when you audition and decide to purchase your'audio' components you listened to them rather then just read the measurements and buy based on that criteria alone. Science is great but it does`nt at this stage have all the answers.

"The human ear is more sensitive than any measuring tool."

My post was primarily in response to the statement above. It should be self-evident that human senses, among the animal kingdom, SUCK!! we build machines all the time, to see, hear, smell, and detect things that human senses cannot. We have systems on submarines that can ID other subs by the sound they make. I am sure a machine can be built to ID every amp on the planet, based of that amp's output of a known input. But, who would want to do that? My post may have been an over statement, but if people say they can hear wire and amps and all the rest, FINE! Just leave science out of it.
Just food for thought, your family pet 'hears' more of your high-end system than you do. :) Maybe we need a K-9 audiogon hahahahahah those would be some interesting threads!!
Peace.