how can a line cord affect frequency response ?

Twl...Most of the parameters which are spec'd for audio equipment are good for verifying that the circuit is working properly, but relate only imperfectly to sonic quality. However, there are tests that one can perform which I think would satisfy your objection. One such test is to observe (by oscilloscope or by listening to it) the difference between a signal waveform going into the device and what comes out, adjusted for gain of course. This signal can be any waveform, music if you like, although other waveforms like a square wave may present more of a challenge. Note that I did not say to measure the difference, because quite disparate waveforms may have similar measurements.

For another example, if you drive two identical power amps with the same signal you should (ideally) see nothing when you measure across the two hot output terminals. Note that this can be done with real music and driving real speakers. Now put an aftermarket PC on one of the amps. If the measurement is still zero (or the same tiny number) it is safe to say that the PC did nothing. If there is a difference you could make a recording of it and play it back so you could listen to it, and form some opinion of the sonic effect of the PC.

hi tvad, am i going to have the last word about my background. it is obvious that i am not a member of the legal profession. i have said so many times. the phrase was used in conotation not to meann, literally that i am a lawyer.

i detect a lot of hostility and you may need to develop anger management skills.

considering the subjects discussed, the relative unimportance of the subject and the banter that has been exchanged to date, i think you need help.

would you like me to provide the name of therapist ?

Dpac996,
I was joking about the "negative feedback power cord", because of the past history of what happened in measurements to product that end in audio amplifiers during the 70's and 80's.

I'm actually pleased that you are interested in doing things to help us understand what we want to know.

I only qualify that with the hope that the tests are done in such a way as to ensure that past mistakes are not repeated.

Eldartford,
I'm glad that you are looking at ways that could advance what we know about testing audio products, and making those tests as relevant as possible to the real-world operating conditions of the products.

I feel that doing those kinds of tests, and even coming up with newer test protocols to accomplish this would get us further toward the truth about what we test and hear.

While I've been accused by others(not you) of being "unscientific", I actually have no beef with science. I only want the tests to be as accurate as possible in determining the outcome, so that we can ensure as far as possible that no incorrect conclusions are drawn from the tests, as has happened in the past.

My aims are to reveal as much truth as possible, and not obscure it.