If you set a sub to LFE only, it will only play the LFE (.1) channel of a multichannel music/movie. Where no LFE channel is present (2 channel music or 2.0 movie), the sub will not be used. In either case, bass from the surrounds and centre should be redirected to the mains.

Roon Developer and ex-moderator of this Forum
I am #25 in the HH1 photo of fame.