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

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 
 

Mapman,

atmasphere stated:
" The room will have some influence certainly, but if you really want 20Hz response **with any sort of dynamic range**, its not going to be a small speaker. Note emphasis."

As far as a speaker being relatively flat down to 20hz, with 'any sort of dynamic range'.  I completely agree with Ralph and I seriously doubt many even exist, certainly no small  speakers/subs.  You're chasing a ghost and I think you may know it.

As far as 'the room will have some influence certainly'.  I completely disagree with Ralph on this because he is seriously underestimating the effect  the room has on bass response and has chosen to ignore the mountain of scientific data compiled on this subject and the single solution that has been proven to be successful in evening out and improving bass response in any room(Distributed Array Bass systems).  The room will be THE MAIN influence of how bass is perceived.   Even if your room is as big as a football field, and you have only 2 spkrs/subs along the front wall (and even if they're launching perfectly flat waves at 20hz from  these  2 points) the extremely long 20hz sound waves will reflect off of room boundaries eventually and cause peaks and nulls at specific positions in the room.  Given the exact room dimensions l x w x h and the boundary surfaces' materials, acoustical engineers can determine exactly where you will have peaks and nulls.

Sure, if the spkrs/subs and the listening position are kept in the middle of this huge space and away from room boundaries, these affects will be mitigated significantly but bass response will still not be even throughout the entire room.  The smaller your actual listening room, the more peaks and nulls will exist.  No amount of room treatments (bass traps, absorption panels and diffusing panels) or room analyzers/corrective sound processors (the ones that use microphones at the listening position for measuring and then equalizers to adjust frequencies to remedy inaccuracies like Room EQ Wizard and all the other systems built into receivers)  may be able to help a bit at 1 specific listening position but do virtually nothing for the numerous other positions in a room.  

Only multiple subs (experts say the more the better but a minimum of 3 subs) strategically positioned at setup process determined locations within a room will remedy bass peaks and nulls in a room and each room is unique since every room has different dimensions.  Trying to do this on the back-end with bass traps, panels, mics, analyzing software and corrective equalizing  (with sub locations initially selected haphazardly and literally at convenient open locations along the front wall) surprise, surprise, definitely does not work and is like trying to put out a fire with a squirt gun.....Go Figure!

  I apologize if I sound like I keep beating this horse/drum but the subject has apparently struck a nerve with me and I feel the ned to tell my story.  About 10 yrs. ago,  I  spent countless  hours experimenting with  positioning 1 and later 2 subs trying to get good bass response in my medium sized room with mixed results.  My first breakthrough occurred when I moved 1 of my 2 subs out into the room between my listening chair  and the front wall about halfway down the left wall.  I didn't know it at the time, but I had accidentally stumbled upon my own limited distributed array sub system.  Later, after months of internet research on how to achieve good home audio bass response, I discovered research white papers and articles from Dr. Earl Geddes, who has a PHD in Acoustical Sciences and currently is the  CEO of GedLee LLC which is a consulting firm that specializes in acoustics, audio, home theater and noise control.  This info concerned his research into achieving good bass response in business and home environments for both music and ht.
 I  have also learned a lot by talking and corresponding with James Romeyn of  James Romeyn Music and Audio in Utah.  James studied under Dr. Geddes for several years before opening his own company that trades in musical instruments and is a dealer for Audio Kinesis.  He is the person who convinced me to buy the Audio Kinesis DEBRA distributed Array Bass system and give it  try.

My main point is that my search for a good home audio bass system was not as simple as walking into an Audio store and buying a distributed array bass system.  It  has been more of a long journey that culminated with a satisfying discovery and excellent solution.  I'm writing this reply to illustrate that I've been at the same stage that many Audiogon members seem to be at; that is, posting on this forum searching for answers to their system bass performance woes  and seemingly believing the solution will be to just buy a certain brand and model sub, plop it down at some convenient open space along their system wall, and their system will suddenly have good bass response.

I know you have been a member here for a long time and  know, through encountering and reading many of your posts over the years, that you have a wealth of knowledge on a variety of audio subjects and have always communicated knowledgeably, articulately and conveyed relevant and helpful info in your posts.  

Precisely because of this, I was surprised you posted what you were in search of on this thread.  I thought you would understand the futility of what you were in search of. I was fairly certain that you knew very few speakers are capable of flat response down to 20 hz and the likely hood of finding a relatively small speaker/sub capable of doing so would be  approaching impossibility.

In conclusion, my current thought is that you posted your question  just to have a little fun by finding out what kinds of replies your paradoxical question would solicit.  

  Am I correct?

Thanks,
 Tim