I have not been following this thread and fear I have stepped in the middle, very sorry for the churn.
"Recommend" is too strong -- it depends upon what you are after, why you are testing, and your end goals. If I want to compare midrange, arguable what makes most sense in this case, then yes I would put a HPF somewhere before the power amp(s) so it is applied to both speakers. That way you keep the source material essentially the "same" and do not stress the LF range of the smaller speaker -- assuming in the final system that is how it will work. How high depends upon the speakers, what you are wanting to compare, how you intend to use them in the system, etc. "Same" in the sense that you can play pop/rock, jazz, a symphony, or whatever and not worry about the results being dominated by the extended LF range of the Salon2's.
Again, I am assuming the Genelecs don't reach as deep, something I do not know. I am also assuming you want to see if the speakers are comparable within their nominal performance range rather than going beyond that range. If the Salon2's have an extra octave or two of bass, and you want that, why do the comparison? Or you want that and will use a sub to get there, then either compare the region above the sub, or audition them as they will be used with the sub. It's your test, you get to make the conditions, and can ignore the faceless Internet posters (me).
As far as limiting the response, a somewhat gross analogy would be to compare power amps. If you need 300 W to achieve your target SPL there is not much point in comparing a 300 W amp and a 50 W amp. Maybe a 300W amp and a 500 W amp... But if you have a pair of identical 300 W amps and are debating using them to bi-amp versus picking up a 100 W tweeter amp, then limiting the response to HF only and power levels to those appropriate for the tweeter makes sense. If the HF section will never see more than say 50 W without exceeding tolerable SPL (or damaging the tweeter), then the test may find the higher power of the 300 W amp leads to excessive hiss or whatever.
HTH - Don