They like it! Now prove they wouldn’t have liked this other thing

I have done extensive research (1:1 interviews, concept testing, and validation) that shows employees like to be paid based on X, Y and Z variables. Now leadership wants to know for certain that employees wouldn't be just as satisfied if we only paid based on X and Y (same net, though). Ideas for how to do this?

One catch - I can't do a survey.

My plan is to,

A) show the enthusiastic response to Z we saw in the research, and

B) do a few lightweight sessions using the same method I used to prove they liked X, Y and Z - but this time with just X and Y. Subjectively assess their acceptance / enthusiasm for XY, which I expect to be lower than it was for XYZ.

I would like to have more concrete, less subjective proof, but without the ability to send a survey and get satisfaction ratings for each variant independently, I'm not sure how to do this...I welcome any suggestions!