Tom touched on an item as to why you must do blind tests if the differences are small -- the placebo effect. Replace your 16ga zip cord speaker wires with megabuck boutique cables and give them a listen. Amazing! More detail, better imaging, firmer bass, etc. Or is it? I'll give you a dollar to a donut that in a DBT, you won't be able to pick between them.Why double blind rather than single blind? Well, the operator can give you clues as to which item is under test, either inadvertently or on purpose. Remember the Coke/Pepsi taste tests? The tester gave the testee the other guy's product first, then his, knowing that the testee would usually pick the last item offered. One would presume that there is no such collusion at an audio club meeting, but single blind offers the opportunity for inadvertent giveaways.
Now, why shootouts don't work. Have you ever gone into a hifi store, auditioned something that sounded wonderful, but was absolutely unlistenabe after the second CD/record? The salesman biased your selection by playing HIS selection louder and with smiley-faced EQ. In the short time you listened, all of the psycho-acoustic clues brought attention to HIS selection, but you were not given enough time for your ears and brain to recognize that the audition was unfair.
Now, I am not suggesting that anyone here at ART would intentionally bias an audition, but the audition time is going to be too short for your brain to process what your ears are hearing, and again, the environment and music selection will not show each of the participants in the best light. You need at least a half hour to get a fair handle on a system. So, sit down for a while and listen. Play your favorite CD's. Get a good feel for what the system sounds like, then move on.
You don't need a base level to compare, you already know what the sound is that you would like to hear. You have to listen to a system long enough to see how it compares to the standard locked in your memory.
Bob