If you will usually be using a subwoofer, then you should demo the mains in conjunction with the sub. I say this because, if you demo the mains and evaluate mid bass frequency responses below your normal crossover frequency, then that mid bass assessment may be nullified once you go into X.1 mode. Since I alway use a sub, I
find it more important to evaluate how well the mains work with the sub.
For instance, if brand X gets a slight edge in your audition because it has fuller sounding mid bass in 2.0 at the expense of slightly flat highs, then that mid bass advantage is eliminated one you enter 2.1 mode. You then have no mid bass advantage and slightly flat highs. Otherwise, Brand Y that has clearer mids and more detailed highs may be a preferable choice if the sub is usually doing the heavy lifting for the mid bass and lower ranges.
As an example only
) - For purely 2.0 music, I might
prefer my previously owned Wharfedale Diamond 10.1's because they have some additional artificial warmth & punch in the mid bass range that would help compensate for the lack of deep bass from a quality sub. In this instance I would be sacrificing the detailed highs and clearer midrange that are generated from my Emotive Airmotiv B1s. Without a sub, the B1s are accurate, but not necessarily "full" in the mid bass range. To some the B1's accurate 2.0 mid bass performance may sound less full than speakers that are voiced to have additional punch. By combining the B1s with a quality sub, I get deep, punchy bass plus
the benefits of more more clarity in the midrange and detail from the highs.
Since the subwoofer crossover is usually set at least 10 to 20 hz or more above the lower +- 3 db point of the mains, 80 hz is probably a fair starting point for auditions. It is more important to set the subwoofer trim to an appropriate level for each speaker tested. Find some jazz or blues tracks with walking basslines that reach fairly low. If the deepest bass notes are thunderous compared to the higher bass notes, then your subwoofer trim is too high. Once you reach a natural trim level for the sub, you can then experiment with crossover points from 50 to 80 hz.