Acoustic treatment

Oh and Mr Rocket science. How big would a helmholtz resonator have to be to resonate at 20 Hz? Anyone care to guess?

It is easy to Dampen frequencies 250 Hz and up. Doing so creatively can make a big improvement in imaging. Unfortunately as you go further down especially as the wavelengths get longer than 100 Hz it becomes progressively harder. I have played with tube traps in two installations with severe comb filtering and irregardless of anything I did with the traps could not get the variation below 10 db. This is walking around using a volume meter and tone generator. In both instances I told the customer that he needed a new room. Back then we did not have room control. Using a good room followed by careful positioning of the speakers and the listening position and damping primary reflections with the goal of improving imaging is the best way to deal with the problem of bass in an enclosed room. Some speakers and sub woofers are significantly easier in this regard. I have never seen tube traps work and my own opinion from my admittedly limited experience is that they are a waste of money. 

Tim, I think you should find a job as a used car salesman, excuse me salesperson.
Actually summed bass below 100 Hz is an advantage in most acoustic situations you would have in the typical home.
Jon, It would be easier with identical subs. But if you have to use the large sub midway between your speakers and the small ones flanking. I would mono the lot. 
Geoffkait  you did not answer the question, how big? Wikipedia has the equation. I'll give you a hint. It is highly unlikely that anyone here has a room large enough to fit a Tube Trap that large.
Back to Tim, The term "room control" is a misnomer. You are not controlling the room. You are controlling the speaker to correct response abnormalities of the speaker/room system, to make both sides have exactly the same response and correct timing so that the sound from each driver hits the listening position at exactly the same time. With a good unit the listening position can be any where in the room! Where ever you plant the microphone. You still have to minimize acoustic aberrations in the room or you will waste a lot of power and even bottom out a sub driver. Any sub woofer system is going to sound better because you can correct and improve the sub inherent frequency response which is hardly ever flat. Every uncorrected smaller (12" and less) sub that I have ever measured has a response curve that falls off as you go down. 
This can be correct and I even give them a response that rises to +3 db at 20 Hz then at 28 Hz falls off like a rock 10th order! Tim, if you want your bass to really perk up you want to make sure each one is exactly the same distance from your listening position. You can put them anywhere in the room as long as the distance is the same. They should preferably all be up against a wall. It also helps if your speakers are also that same distance assuming everything is phase correct. 
                                         
                            

Sorry folks. It falls off like a rock at 18 Hz. My typing is worse than awful.
They did not teach boys how to type because we did not become secretaries. 

Geoffkait. Still not a direct answer. I probably knew what a Helmholtz resonator was before you were born. In 1978 I had two RH Labs (Randy Hooker) sub woofers. They were essentially enclosed Helmholtz resonators adjusted to 30 Hz if I remember correctly. They were by today's standards huge. I do not have a scientific calculator but to make something the shape of a tube trap a 20 hz Helmholtz resonator it would have to be huge. Think of a 20 Hz organ pipe. 32 feet long. Yes I know it is a different principle. Oh, and the equation is

                          F = v/2 pi times the square root of A/V L

F is the frequency, v is the speed of sound, I hope you know what pi is,
A is the cross sectional area of the stem, V is the volume of the chamber and L is the length of the stem.

Oh and geoffkait, I am a physician and we are notoriously bad at math but you keep avoiding the question which means to the rest of us that you have no idea what you are talking about.
My experience with tube traps under the most scientific conditions I could create is not good  and I do not intend on going down that road again. I see it as a waste of money. There are better ways to deal with room acoustics

Boy are you out to lunch on this one Tim. It is not what you hear. It is about what you feel. I would bet that you have never played with a system in which you can control phase and arrival times which is why you have this rather flaky opinion. This makes a major difference in the realism of the system. Just wire one sub out of phase and see what you get. I've got news for you. You have to be in phase the entire cross over region. If you are 1/2 wavelength out of time you have one sub wired backwards. This would be the kind of biased unacceptable opinion you swarm guys would come up with. If you think the bass you have now is good, you have a long way to go. 

geoffkait tell me how something the size and shape of a tube trap is going to modify the dispersion of a 32 foot wavelength (20 Hz) in a 15 X 25 foot room. 

Ok Geoffkait here we go again. Show me solid objective evidence that tube traps work. Before you start spending other peoples money you have to be full of more than BS beside not knowing how to manipulate the math behind the Helmholtz Resonator. And, don't give me manufacturers BS either. How are you going to locate standing waves in a room and exactly at what frequency? What about comb filtering? In the context of what type of loudspeaker?

IMHO the only rational for tube traps is that a lot of people make money on them.

Geoffkait, just throw the CD's away and download all your music in MP3 files. That should be perfect for you. Your ear works just like any other microphone. It detects minute changes in pressure. In your case you might want to clean the wax out first. In the office we use a waterpick for that. 1 part hot water to 1 part  peroxide. Careful not to squirt your eye:)