There's a lot more bass in a 6.5" driver than most of you think

elliotbnewcombjr:"Before, large room, he was thinking of adding a single subwoofer, now not feeling the need. If only 1 sub, I think it would add mud. A pair of small directional subs, say 10", self powered, adjustable crossover and individual volume, could extend the bass keeping it directional."

Hello elliotbnewcombjr,

" A pair of small directional subs, say 10", self powered, adjustable crossover and individual volume, could extend the bass keeping it directional."
     What? Did you just invent a new 10" sub that defies the laws of physics? Do you think you just point them in the general direction of your ears and the bass soundwaves just travel like a beam of light directly to your ears? C’mon Man, bass below about 80 Hz is not even directional meaning we cannot localize it. The bass below about 80 Hz is actually the opposite of directional and is, in fact, omnidirectional which means the bass soundwaves radiate out from the sub driver in all directions at once. The bass spreads out from the driver in all directions at once forming a spherical pattern and does not travel solely in a straight line, directional path as the soundwaves above about 80 Hz do.
     There’s also some other pesky laws of physics to consider, such as the length of soundwaves increase as its frequency descends and decreases in length as its frequency ascends (soundwave length is inversely related to frequency). Also, our brains can’t even perceive a sound at all until a complete full cycle soundwave is detected in the room by our ears and multiple full cycle soundwaves need to be detected before a change in pitch is perceived.
     Based on the speed of sound of about 760 mph, this calculates to a 20 Hz deep bass tone full cycle soundwave being 56 feet long and a 20,000 Hz high treble tone full cycle soundwave being a fraction of an inch long. Given these facts, it’s not very hard to comprehend that the very long and omnidirectional bass soundwaves behave very differently in any given room than the much shorter midrange/treble soundwaves behave in any given room.
     All of the above almost guarantee that the highly omnidirectional bass full cycle soundwaves launched by a speaker’s bass driver into the room, some of them likely being even longer than any of the room’s dimensions, will bounce or reflect off of one or more room boundaries (floor, ceiling walls) prior to reaching and being detected by our ears and our brains processing it as a perception of a bass sound tone. This is true whether the soundwave is launched from a 5" or 15" driver.
     More to the point of this thread, however, it does seem logical to believe that a larger driver would be much more capable and efficient in reproducing the correct bass frequency soundwave amplitude (intensity level or volume level of the frequency) than a smaller driver would. IOW, reproducing deep bass notes requires moving a lot of air. Given identical excursion range distance (the distance a driver is able to move forward and backward like a piston.), a driver that has a larger diameter will move more air than a driver with a smaller diameter will. 
     The only way for a smaller driver to move more air, to equal the total volume of air moved by a larger diameter driver, is for its excursion range distance to be increased. However, the excursion range distance of drivers are not unlimited. There are factors that limit the maximum excursion range distance of a driver and this specific excursion range distance measurement, along with the specific driver diameter, also limit the maximum bass frequency depth the driver is capable of reproducing.

NEWS FLASH SOLUTION: 
     A pair of good quality subs, properly positioned and configured, are capable of providing exceptionally good bass performance in any room and seamlessly integrate with any pair of main speakers at a single designated listening position. Utilizing two pairs of good quality subs will perform about twice as well as a single pair, with near sota bass performance along with providing this bass improvement throughout your entire room and not just at a single designated listening position.

Word to your mother,
Tim

You are talking about many different things in this post: smallish apartment rooms, added subwoofers, and room treatments.
Sure, smallish rooms can better support smallish speakers.
Room treatment in every room can have a dramatic effect on SQ.
Subwoofers can increase SQ of big and small loudspeakers. A sealed speaker system in a larger room will never produce low end energy by itself.

There are many speakers systems with smaller woofers but the better 1’s use multiple smaller drivers instead of 1. Also, the reason why these systems can produce low frequencies is due to the port setup, which can increase lower bass frequencies. I used to have the Usher mini dancer 2’s with multiple 7” drivers and they would produce low levels of bass energy for its size, but not the cleanest bass output. I tuned the port with acoustic fill and added 2 Rel subs. This was in a treated dedicated 27x16x15’ foot. I sold them and went with the larger Ushers with the 11” woofer, much better tighter lower end and no need for subs.

...the interesting thing about line arrays is when all those small speakers combine for bass lines.  If one doesn't get too enthusiastic with the drive levels, they respond quite nicely...

I remember my 901's of eons past....not all that accurate, but plenty of it...;)

Thank you all for participating so actively, especially those who can share personal, specific experiences and who read the nuance in my original post.

I was thinking of something that I think has helped this discussion: I used a DIY speaker as my example, otherwise many would accuse me of shilling for a particular brand. At the same time, I don't mean to imply only this speaker can output this much bass in a modest listening space, I'm sure many others can as well.

 I used a DIY speaker as my example, otherwise many would accuse me of shilling for a particular brand

you used your own DIY designs. And all the effort youve spent making that response curve as flat as a pancake has been a waste because that was the wrong curve. You will need it completely retuned now. We need to examine the off axis curve too.