You don't have to level match. Or follow any protocol other than doing the testing blind. Have someone switch out one product for the other a dozen times once a day and you both keep score of which unit is what. Then at the 12th day compare notes and see how it matches your sighted evaluation.
I may try such a thing. I could use my harmony universal remote so that I wouldn't know which preamp I was using. I'd just need another DAC, perhaps one I find not as pleasing/different like the Oppo 205.Well actually since my DC-1 is an analog preamp, I could just that as the input for a cheap dac and see if I can hear a difference. I have a $30 Behringer 202 which I thought sounded quite good.