During the session all of us involved were utterly convinced that we were hearing the expected characteristics we associated with the different types and makes of preamp. It was a fantastic demonstration of confirmation bias because once the files were anonymised none of us -- and none of those taking part in the survey -- could reliably identify the individual preamps.
Actually it doesn't always goes this way, even when you know this preamp should be better, because of a better name of higher price tag. It happened recently in the studio comparing two preamps including a very big name. Everybody agreed the lesser known preamp was performing better (testing with acoustic steel strings Lowden guitar and breathy wooden flute)... including the very owner of the expensive well known machine.
The practical issue, of course, is in finding a reliably repeatable source with that kind of spectral content.
No doubt about that