What are the smallest speakers that are clean and flat down to 20hz?

I use two Torus Infrasonic Generators. They go down to 20Hz. Two Carbon fiber 18" diaphragms. They have a large heavy core with windings above and below the diaphragm, and work in push pull. Fast like an electro static speaker. No doubling either. These are not your usual long throw drivers with heavy suspension systems that slow things down. These beauties play music. If that matters. 

Two 15" Rythmik subs crossed over by a DEQX 

Two 15" Rythmik subs crossed over by a DEQX 

Perhaps the question should be how to get low frequency that is fast enough to keep up with an 8" or 10" woofer in the main speakers. Not many available. I mean how do you make a large diaphragm move as quickly as a smaller diaphragm. Not easily done. You might move a ton of air with a long throw sub but is it clean or just double like crazy. If you walls start resonate then you are probably doubling. 

Good one (or two ;-) Mark. Very few speaker really play flat to 20Hz cleanly, and the Rythmik F15HP (and it's smaller brother the F12) does so in a small enclosure and at a low price. The best value in high-performance subs!