I'll chime in as a def tech owner with the lfm-1. I wasn't sure I really needed the sub, but I couldn't have been more pleased with the purchase.
It goes considerably lower and has made the soundstage much more clear overall.
My Def techs put out some good bass but it was somewhat boomy and muddled in comparison, and the specs for how low thier mains go are suspect. overall a great combo.
When I move my system to the basement( whenver it get's finished)I will probably purchase a second LFM-1.