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

OP,  Demo the Kii 3s for yourself.  All of the blather will fall away and the music will fill the room.

Does not quite meet the parameters you posted, but very impressive low frequency extension for speaker type and cabinet size. Smaller footprint then the majority of stand mounts. 

http://www.decware.com/newsite/HORN.html
Scroll down the page for specifications.

No affiliation with Decware

Open baffle, down to 18Hz

http://www.spatialaudio.us/x1-main

So bass is one of these subjects that never is settled, as most people will hear and FEEL the sound waves. Perceived bass is
all up to the individual listener to make it what it is. I'm very familiar with the Ohm speakers an "Mapman" knows and every time someone tell me they can hear below 20hz with their (insert model)
I laugh because that's kind of saying you can hear a mouse fart...sure you can, but mouse farts are very, very quiet.

As far as bass I'm using a four 18" driver infinite baffle system
that  will shake my house to the point it's scary and it can put out 120db with no effort at 20hz. My large Ohm speakers 4/5000
will comfortably crank out 25 real hertz all day long but anything below that level is negligible in a true sense and still nowhere close to what the sub can do....

peter

I was impressed with folded horns think I will be getting familiar with some of my own. I was exposed to some low bass I could feel more than I could hear at times, like an invisible donkey kicking you in the chest. Up in the more practical musical hz they slam well and make beautiful coherent bass. If I had the space I would love some giant la scalas with matching folded horn bass' that reach for the ceiling.