I've been using the Rythmik L12 sub for about a week now and so far I find it very close to what I liked about my Rel. Although this doesn't have the neutrik connects like the Rel - a very easy hookup - it integrated into my system easily enough.
Im still tweaking the crossover point and playing with the positioning, but this is a very well made sub, especially for $539 - you can spend the same at a big box store and get far less for your dough.
Imight get the sweet spot this week