Since you are on the Axiom website and I assume you are an Axiom owner, why not consider an Axiom sub. Both the EP175 and the EP350 (which I own) will go deep and clean and work with any main speakers beautifullly. Just be sure to adjust sub volume and crossover carefully to match your mains - a helper and a sound meter (affordable from Radio Shack) are a big help with this. A test tone disk doesn't hurt either.

Hope this helps.