The problem with using real loudspeakers for testing is that of standardisation. For measurements to mean anything, they have to be standardised so comparisons can be made. That's why 4 and 8 ohm pure resistive loads are the norm, they're the same for everyone.
For a reactive load, one can do as the Power Cube tests do, with known inductive and capacitative loads, and that is a reasonably accepted standard. Alternatively, something simpler like the old 2uF 8 ohm, but that has its limitations.
How difficult would it be to make a switchable 2-4-8 ohm load with switchable inductive and capacitative loading?
S.