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

PMC's, which have a transmission line enclosure, can produce more bass for their size than other designs. 

Transmission line makes a speaker larger.

The cabinet on Barefoot MM27 is Cabinet: 20.5 x 9.5 x 15.5 inches (521 x 241 x 394 mm)
Overall: 20.5 x 10.5 x 17.4 inches (521 x 267 x 442 mm)

it is flat to 30Hz and will play louder more cleanly than 99% of what people have here.

I thought the OP criterion was clean and flat to 20 Hz?
Perhaps he can specify what flat is to him: +/- dB ?
If the recording can contain it, and my room can contain it, I want my system to deliver it.

Mapman,

Even if you were to find 2 ch speakers or subs of any size that were flat down to 1hz +/- 0 dbs, the unfortunate truth is that the physics of acoustical sound wave propagation is going to ensure you don't perceive flat bass response in your room, assuming your room has walls, a floor and a ceiling. As long as your spkrs/subs are located along your front wall, there will be uneven bass response in the room; specific spots in your room that bass is exaggerated and specific spots that bass is diminished and even nonexistent.    

Sorry to rain on your parade but I've discovered there is a good solution, called a Distributed Array Bass System, that works amazingly well. These systems are sold under various names (SWARM, DEBRA, etc.).  I have used the DEBRA system for about a year now and I'm completely satisfied with the results. I started an Audiogon thread in June of this year that goes into more detail and requested input from other users (there doesn't seem to be many Audiogon users that employ this bass system).  Here's a link if you're interested: 

https://forum.audiogon.com/discussions/anybody-else-using-a-distributed-array-sub-system

You can also google "distributed bass array system" that will list multiple sources of independent info, including sites on the specifics of how it works, white papers that discuss the theories behind it with empirical data as well as current sellers of these systems.

I should mention that I was initially very skeptical due mainly to the cost ($4,000) and whether it actually worked as well as claimed.  It took months of emails and phone discussions with the seller before I decided to give it a try.  I am now very grateful I bought this system that works so exceptionally well for both my 2 ch music (especially on high-res music files that are thankfully without 'loudness wars' dynamic limitations) and HT. This system has given me solid, deep and tuneful bass that sounds equally good from all 6 seats in my 23' x 18' room.  It is somewhat expensive (I rationalized it by comparing it to the cost of 2 high-end powered subs I was previously considering) and it takes the better part of a day to properly setup in a rather detailed and extensive process.  But I now consider it a bargain and realize the extensive setup process is critical for good results.  
Lastly  a disclaimer, I have absolutely no affiliations, financial or otherwise, in the development or sales of these systems.

Just a very satisfied customer,
    Tim 
 

Good stuff folks, keep it coming.

noble I just want to know the small speakers capable of it in a typical room.

I’m familiar with the swarm approach. That’s probably the most effective approach in the end if done right I’d say..

30hz isn’t bad as long as the size is small accordingly. I could probably live with that. But 20hz or even lower maybe is best.