My thought is that repeat testing should use a variety of hops that have high and low cohumulone levels, this way a trend would tend to point towards cohumulone as the likely difference. If you use the same two hops repeatedly, then the results could be from some other difference between the two.
I think you'd want enough bitterness to be able to detect a difference if there is one, but otherwise as small of a hop addition as possible to minimize other flavor contributions. In other words, high alpha but low oil content would be the ideal hop selection for this.
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