I don't know that I have ever appreciated a change in the midrange of my system with the subs active.When done properly, i.e. not using idiotic full range pass through the sub but a crossover that removes the bass & possibly some of the mid-bass from the mains, the LF driver has a fair portion of its work removed, allowing it to better handle the upper part of its range. The mains' power amplifier loafs, having gotten an effective 2x+ power increase by removal of the lower registers.
I'll wager dollars to doughnuts that six nines have never heard a properly integrated sub.