Hop, I really like your approach on this and the results should be very telling. While your doing the test could you structure the the questions to something like...
Q: with your back to me and not seeing which instrument I'm playing do you hear a difference between instruments 1, 2, and 3 while I play them with all amp settings on neutral and with each instrument volume knob set at the same percent on? If you hear a difference please describe the difference ( i.e. tone, attack, sustain, etc.).
Q: now while you play each instrument unplugged, do you hear a difference between instruments 1, 2, and 3? If so please describe.
Q: now while you play each instrument with whatever amp settings you most frequently use, do you hear a difference? If so describe.
Q: Please adjust the tone, pickup, and volume settings on each instrument until it sounds best to you? Did you end up with the exact same settings on each instrument? If not, why?
Q: using the following wood tone descriptions (lifted from the Warmoth website) which instrument would you say is most likely mahogany, which one do you think is alder and which one is maple? Please indicate the degree of certainty with your selection of which instrument is which wood using a scale of 1 to 10 with 10 being the most certain and 1 not being certain at all.
Now that would be an interesting test!