If you're running a subwoofer to help achieve a truly full range sound, it might make sense to feed it with a dedicated subwoofer output on your preamp.
But, there's a s problem. A modern high-end preamplifier hasn't a subwoofer output.
Why?
Any decent subwoofer has its own crossover built in. In fact, I can't think of one that does not. That crossover was specifically tailored to that subwoofer and it's always going to be your best bet for setting its low pass filter to the highest frequency to seamlessly mate with your main speakers.
Where did this idea of a subwoofer output come from?
To the best of my recollection, the subwoofer out is a holdover from the era of receivers and cheap subs for home theater. Back in the earliest days of subwoofers they were either unamplified or had full range plate amps driving them. In more modern times, home theater receivers contain all the DSP functionality and it probably makes more sense to use that integrated system instead of trying to mate the subs yourself, thus the separate subwoofer output on receivers.
But this isn't a home theater forum.
When it comes to high-end 2-channel audio, the best way to feed a sub is a plate of full range audio from your preamplifier and let the sub's electronics do the rest for you.
