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

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!

I’ve been considering adding subs to my second system that uses smaller limited range monitors and Rhythmik subs are the leading contender if I do. I used to use an older M&K 12" sub there that gave up the goat a couple years back.

Not much happening there below 50hz or so currently but what’s there does sound quite lovely these days indeed and my other main system covers the bases that one does not pretty well. its nice actually having two different windows into the music, one with good authorative detailed and articulate bass extension down to at least 30hz or so that one can feel as well as hear, the other just nice and clear and detailed music to hear if not so much feel.

Then there are the headphone options with these as well for yet a different way to listen but that's another story altogether.

Yes, about 40 Hz should do it for most music (unless some crazy person uses real cannon in an overture)

and 3.7 Maggies will do a decent job on 40 Hz or so - and... it you think about how thin they are, maybe they are the smallest...