My main speakers are MartinLogan Prodigy’s, and I still run them full range. I don’t use the speaker level crossovers in my subs…they are essentially just connected in parallel to the Prodigy’s.
One of the things I struggled with for years was getting the subs to “track” properly when changing volume. In other words, I never felt that the bass was optimal at all volume settings, meaning I had to set up the subs the way I liked them for the volume at which I normally listen, but then they were a little “light” at lower volumes, and slightly boomy at higher volumes. Using the speaker level inputs, I don’t hear that issue, and I get what I consider appropriate bass response at any listening level.
Also, even though Paul said that the additional “loading” of the preamp from the interconnect driving the subs “shouldn’t” matter, in theory it still does present a different load impedance to the preamp. My general thought would be that the output of the preamp would be more sensitive to changes in load impedance than the output of my power amp (Krell FPB-200), so relieving the preamp of having to drive a long single-ended interconnect cable to a subwoofer should be a good thing…
I know Paul has written about using speaker level inputs for subwoofers as the preferred method, I just don’t know where I read it.