In reviewing, never. It is not a pure comparison if one uses another device/product, component, cable, etc. if the idea is to gain insight how speaker A compares with speaker B. Any additional element will skew the result. I add that the comparison might be of the two speakers in situ, or might be as maximized, which would involve typically different setups. I enjoy comparing ultimate performances of products, as it might be expected that when a different speaker is dropped into the system that has been optimized for the initial speaker, the new one would be expected not to perform at its potential.
Having said that, it is legit once a baseline is established, to add a subwoofer, etc. Obviously, the comparison is then between complementary products, sub and bookshelf, vs. floor standing speaker. Results will obviously change, and that’s ok.
There is no sin in working with subs and bookshelf speakers, however it should be remembered that typically the full range/floor standing speaker will also benefit from the subwoofer(s), so there’s often no net advantage in employing the sub(s) as though it really gets you a radically different outcome between the bookshelf and floor standing speaker - unless, of course, you ignore potential use of the sub with the tower speaker...
Having said that, it is legit once a baseline is established, to add a subwoofer, etc. Obviously, the comparison is then between complementary products, sub and bookshelf, vs. floor standing speaker. Results will obviously change, and that’s ok.
There is no sin in working with subs and bookshelf speakers, however it should be remembered that typically the full range/floor standing speaker will also benefit from the subwoofer(s), so there’s often no net advantage in employing the sub(s) as though it really gets you a radically different outcome between the bookshelf and floor standing speaker - unless, of course, you ignore potential use of the sub with the tower speaker...