Thanks Guillaume! To me it would be easier to do for example A/B testing if the levels wouldn't change. Levels, as we know are crucial, since the louder one almost always sounds "better" even though it's a whole different story when the levels are adjusted to be equal.
I've been using also the Ignite Amps NadIR Convolver and it has this handy Balance control with which you can blend between two IRs and the level stays the same. I understand that WoS is much more complicated thing, and that kind of control probably wouldn't work since you can load more than two IRs in it.