I have not had an opportunity to own the REL sub however I have maintained Velodyne DD18's as well as the JL Audio Fathom F212 V2. The JL is a much quicker response sub due to the smaller driver however the DD18 is no slouch. Both excellent subs that I have incorporated in two seperate systems however I sway towards the JL. Using the JL with a pair of JBL Array 1400's which blend perfectly.
