Assuming I have my sub set-up correctly, the REL should be connected via both R/L amp outputs and LFE connections. The left and right speakers should be set to large, and the sub will effectively operate full range. For LFE and bass redirection from your centre and rears (anything other than R/L), the sub should obviously be line out.

Yes, Trifield is a bit thinner than music in the bass, but it shouldn't feel inadequate if the sub is set up right.