No-one uses a blindfold. And that's a very poor way to control an experiment.
If two things sound different, they will still sound different if you don't peek.
I'd run the study this way:
- Experimenter A sets the two equipment chains up, levels them, and obscures them with screens.
- A pseudo-random digital gadget runs a switch between the audio gear being tested.
- Experimenter B briefs each subject with a quick script.
- Subjects enter the room one at a time and sit in a designated chair.
- Subjects record the results of a series of n random listening sequences on a sheet of paper.
- Experimenter B asks for impressions not captured by the in-room questionaire, post listening.
- some means is provided to detect Subjects that peek behind the curtain or engage in other disqualifying behavior.
Ok, so you still have to pick listening material and how that is handled as a cross-variable.
Impression data is correlated post-study with the known pseudo-random sequence for a particular subject.
This seems to meet criteria for a double blind study. I'd be really careful not to leak information about what equipment is studied,
what the experimenter's opinions are, etc. I'd also design the approach to the study (the physical approach) to be as neutral as
possible, (i.e. no 'priming' audio equipment in sight, logos, etc.).
This is all pretty standard behavioral study stuff, avoiding "Clever Hans" effects, "priming", information influences subject to subject and
ensuring it really is double blind.