What I have found is that adding the Rels makes the entire soundfield bigger, more dense, more full, adding to the PRAT, and actually Improving the level of Detail that I hear in all my music. Yeah I know, you've heard that last one before, but it's True! Once the Rel's settled-in I began to hear intricate details and subtle cues in the music that I had not previously noticed.
I don't know or understand the physics, but adding the two Rel's took my rig to the next level! I seldom run the Rels above 28hz or 30hz and always at a very low volume level (typically less than 10). It WORKS!
I prefer the Rels over Magnepans own supplemental woofers beacuse they do not add an additional Load to your amps; the Rels have a very high impedance so they dont add a burden to your main amps (which are already working overtime to drive the Maggies).
The Rels make an Excellent speaker, the Maggie, an even better one!