I would argue that riding the volume up during quiet passages, or any manner of playing the material at abnormally high volume for the purpose of pushing the distortions into audibility is cooking the tests.
As for blind versus double blind, I think that single blind can be adequate in many cases. I've tested people blind in my own studio to my satisfaction. I knew which file was which, but the listener didn't and I'm confident my facial expressions gave nothing away. The last the I did such a test I was facing away from the listener. I also test myself blind by closing my eyes so I can't see which state the solo buttons are in.