I had a Paradigm Seismic 12 and found it to be good as a movie "boombox" but not even worth considering for integration with my 2-channel system for music listening (using Thiel CS5i's for main speakers). I do agree that Paradigm makes excellent speakers, but this sub was obviously designed to be used strictly as an LFE channel sub in HT applications.
I replaced it with a REL Stadium III which was designed for 2-channel integration with music sources. After a ton of experimentation with positioning, crossover settings, gain settings, and, most importantly, adding massive sealed bass traps from floor-to-ceiling in the front corners, I finally got the REL to integrate (nearly) seamlessly with my Thiels.
Additions of a Synergistic Research Tesla REL Spec II hi level cable, cones spikes, and a heavy gauge power cord with Rhodium plugs made further improvements in the REL's performance in my system. It now adds a proper yet subtle bass foundation to the Thiels and magically enhances the soundstage depth, layering, and realism, which has proved to be addictive.
I have only heard the JL Audio subs in dealer demos but they seem to be excellent subs. The advantage of the REL in some 2-channel music applications is that they use a hi-level interconnect from the power amps left/right speaker outputs directly to the sub, eliminating the crossover in the main speaker signal path. This works well with speakers that already have a strong bass frequency response that requires no equalization to provide the desired tonal balance in your system. I suspect that the crossover/equalizer in the JL Audio subs would be beneficial to main speakers that need substantial bass reinforcement and/or have problematic bass linearity issues where room treatments are not an acceptable solution.
