REL recommends running them from the speaker posts; I run mine that way. Seeing the output of the amp gives the sub a chance to mimic the output of the main speakers according to REL. But As Always the best choice is to try both and go with the one you like best.
REL subwofer High Level vs Low Level connection
My preamp has two sets of outputs giving me the option of running interconnect to the sub's low level inputs or running the Neutrik-cable from the speaker connectors to the high level inputs. I would think that these are very different solutions electronically and with potentially very different sonic results. Thoughts anyone?