Summary: You're not testing the performance of the output of the soundcard, but also that of the input!
Finally something useful and informative after all that previous ranting. Well, this is also ranting, but I have a valid argument.
Loop-back test, people use it to test the performance of a soundcard.
It is useful, for example in recording, to test the performance of the system.
DAC out to ADC in.
That is where the problem is.
You're not just testing the performance of your output, but also the performance of your input. Actually to be precise, you are testing the loop-back performance of your system - how much the sound kills itself when you play it and record it back.
And most soundcards have better DACs than ADCs.
So the ADC becomes the bottleneck, the decider of the score, not the DAC.
So loop-back tests almost always become a test of the input, not the output.
And how does that result matter to people who are buying soundcards just for playback, and choosing soundcards based on output performance?
It doesn't matter a hoot does it.
So, RMAA loop-back test as a measurement of soundcard output performance - Invalid.
How should it be done then? Simple, output to a soundcard with better inputs.
And I'm not the first to notice this - there is an example here:
http://ixbtlabs.com/articles2/multimedia/m-audio-audiophile192.html
Loop-back tests do have their use. For example, calibrating your mic and speakers of your recording system. And loop-back sound quailty.
Just not really as a measurement of just output sound quality.
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