tompoland My espresso/grinder experience is certainly less extensive than yours, having really only started drinking espresso on a regular basis since last February, and having owned a whopping two grinders. But I know a fair amount about clinical trial design from the day job, so maybe this will be of help.
You’ve done a good job isolating the variables that may influence your observations between two grinders. If you’re going to be recording subjective impressions, there is a standard approach to identify differences based on subjective measurements/observations.
First, you do several runs of blinded testing with each grinder comparison. The most important thing is that during this process, keep the identity of the grinder secret while doing this.
After you’ve done that, your wife can reveal which cups and observations were from which grinder, so you can group your subjective observations. Then look for thematic consistencies in each group of subjective observations.
For example, if you observe “fruity notes”, and that description consistently shows up in the observations from one grinder, then that indicates that there’s something about that grinder that brings out fruity notes. But if that description is seen in both sets of observations, then it’s not likely to be due to a difference between grinders.
And if you want to be really rigorous about this, your role would be just to record your observations. After the observations are unblinded and grouped, multiple people besides you (your wife and her friends?) review the grouped unblinded observations to see if there are themes that show up in one group and not the other. What you’re looking for is agreement among multiple people that the “fruity note” observation is consistently associated with one grinder and not the other.
As a matter of fact, if you take this approach, you may not have to do the “make two espresso shots simultaneously” thing, which may make things a lot easier. You could have your wife grind the beans with one randomly chosen grinder, and hand you the end product without letting you know which grinder was used. Then you make espresso and record your observations. With this, you can do this at your own pace. Keep doing this until you have several observations for each grinder. Then your wife reveals which observations were from which grinder, and then do the thematic analysis as I described.