Free air resonance

Try BassBox Pro.

@hsbrock , That is what ported closers are all about. It is not the kind of driver I am use to dealing with. A BL of 31 is a very potent motor. It sounds like more of a driver for infinite baffle designs.

@unreceivedogma , That is exactly the point. You want as little baffle showing as possible. I just finished building new subwoofers, They are 30 inch long, 15" in diameter with a 12" driver stuck in both ends. The construct is easy. I did it for different reasons as subwoofers do not image. 

@unreceivedogma 

Actually, I would modify that design to make the baffle 18 X 18" decreasing the size of the face. A tube would be perfect. You can buy aluminum pipe in that diameter. This assumes a cutoff at 100 Hz. It would be like the driver suspended in space. No enclosure effects. An appropriate stand could hold that at ear level no problem. 

I suspect the crossover is second order. They are shelving it to keep higher frequencies from dominating the bass, a high shelf filter. This is probably due to baffle step, a problem my design above will not have. 

You would close the port and epoxy the panel in place to keep it from resonating, only if you are dedicated to subwoofers. 

My goal would not be to maintain a period presentation, but to take the driver to its maximum performance in term of sound quality and image. It is the very rare system that can cast the best possible image. Images of all instruments and voices should be well delineated in space with blackness between. They are hung in space as if standing there. In most cases, particularly high frequency instruments like cymbals, items are bloated and run into each other. Close your eyes and listen. The analogy would be with light. Shine a naked lightbulb is a room with white walls (reflective) and the whole room and everything in it will light up. Shine the same light in a flat black room and only the the objects within will light up. Theoretically you could get the same effect in a darkened white room with individual lights focused exactly on the objects only. Hard to do for sure, same for sound. It is very difficult to get the sound focused on the individual instruments. This is why the speakers and room have to be considered as one transducer. All this assumes the best recording and mastering. The other problem is that the recording and mastering are being done by people who are listening to their own problems. In any case you would be surprised how good a system/room can image. Line source dipoles are so good at this because if you absorb the sound coming from the rear they are "shining light" only on the instruments and not the room. The problem with omnidirectional speakers is they are the naked light bulb in the white room.

@unreceivedogma 

Great. I'll bet they are plywood. The older manufacturers used plywood. MDF did not become available until the late 60s. Plywood is stiffer than MDF and will resonate at a higher frequency. It is also lighter and not as well damped. It is also way more expensive. Look at the back of the cabinet. If the rear panel is held on by screws it is for certain plywood.  J Frum is correct in what he says from a modeling perspective but the equations assume an infinitely stiff structure in an anechoic chamber. They can not tell you what any given speaker is going to sound like in a real environment.

If I had those drivers I would build 10 cubic foot sealed enclosures 20" X 20" X 40". I would mount the driver in the 20" X 20" face and create a stand that held that face at ear level. I would cross them to subwoofers a 80-120 Hz and control the whole show with a digital preamp. Making it look good would be a challenge

In your case I would block the ports from the rear with plywood and cross to TWO 15" or FOUR 12" subwoofers using an active 2 way crossover. If you really wanted to get the absolute best out of the system and be able to tune it exactly to your taste get a digital preamp like the new DEQX Pre 4, Trinnov Amethyst or Anthem STR.

If you are asking why the strange shape (my enclosure), mounting a driver like that increases the stiffness of the baffle and places the mass of the enclosure in line with the driver minimizing the effect of Newtonian forces created by the driver. The small face minimizes enclosure effect which should improve the imaging. 

@unreceivedogma 

I could not afford them either. Just happened to be at the right place and time.

I read an Altec spec sheet on the Duplex and the 3dB down point was 60 hz which would be about right for a woofer of that type of construction. 

T-S parameters (Thiele/Small) are a set of parameters that define a driver's electromechanical behavior. From them you can calculate the volume of the enclosure you need to obtain a given type of performance. They help getting you in the ballpark when designing a loudspeaker.  https://en.wikipedia.org/wiki/Thiele/Small_parameters

In order for me to get a handle on your enclosures I need the interior dimensions and the material they are made of such as 3/4" plywood or 1" MDF. The type of enclosure, sealed or ported and the dimensions of the port or ports. In today's world of loudspeakers, with the advent of DSP you can get away with murder. I design and build my own subwoofers and rely on digital signal processing to get them under 20 hz at + 6 dB. I am about to shoot the finish on my new ones and will publish pictures of the entire process on Imgur when I am finished. The best way to handle subwoofers is with digital bass management which is antithetical to the type of system you have or rather the type of attitude you have. It certainly is more of a challenge and I understand the attraction. You can push the envelope a lot further. You can high pass your  Duplexes by putting the right value capacitor in series with the input of your amps. You have to know the input impedance to calculate the value. Then low pass the sub with the filter it has, the highest it will go without calling attention to itself. With two subs I would definitely plug the ports but I have to warn you that I really dislike ported speakers. I would rather push the speaker down with digital bass management and use subwoofers designed specifically to make low bass. What you have in the Duplex woofer is really a very large midrange driver. Middle C is 254 Hz. In it's day it was a very fine driver and still is. Back then subwoofers were nonexistent.  

@unreceivedogma , AHH the Duplex. I was thinking Voice of the Theater. Nice trick with the burlap. Yes, I think a rug would be in order.  I beat you by one year. I built my ST 70 and PAS 3X when I was 13. A friends father gave me a pair of AR 2ax's which he used at a party and wanted to get rid of. He owned an electronics firm in Boston. 

Your woofer crosses to the horn at 2 kHz and cuts off at 60 Hz. I can not find the T-S parameters for the 604a,b or c. I would treat it like a Bozac woofer with a large sealed enclosure on the order of 8 cubic feet. They are crying for custom enclosures. IMHO you need two subwoofers one next to each main speaker crossing as high as possible to get the low bass out of the Duplexes. It will make the Duplexes so much cleaner. I have never heard the Futtermans but I use triode OTLs and I bet they are a great match for the Duplexes. They are another reason though to take the low bass somewhere else. Are you high passing the Duplexes now? You mentioned custom crossovers. 

@erik_squires I agree entirely up until the last comment. This argument between wide dispersion and limited dispersion has been going on since I was knee high to a grasshopper.

Obviously, wide dispersion bounces sound all over the room creating more and louder reflections. Very limited dispersion, like a flat panel ESL is very annoying but sounds terrific if you lock your head in a vice. Controlled dispersion on the order of 45 degrees like you might get from a horn system or curves panel ESLs is best in terms of image and comfort. Dipole line sources are even better because of  strong attenuation to the sides, floor and ceiling.

I think it helps to think of sound like light. If you shine a flashlight at an object only things in line with the object will light up but switch on a naked bulb and the entire room lights up.  Omnidirectional speakers "light up the entire room." This usually makes a system very bright which some people like especially at low volumes. You may note that cymbals are poorly localized and there is a tendency towards sibilance with female voices and violins. A system with controlled dispersion usually sounds dull at first and people will think there is no high end. if you pay attention you will note that the cymbals are well defined and the high frequencies are there but now they are coming from the cymbal and not the entire room. Such a system is smooth and effortless without any sibilance. With loads of sound deadening you can make an omnidirectional system sound more like a controlled dispersion one at some expense and nail holes. 

@unreceivedogma , What is rock wool? I have not heard that term. Sounds like you have a nicely dampened room which is great because Altecs  have a tendency to get shouty. 

I think you are overthinking things. Use the pair of drivers you think are best constructed and listen to the results. Stiffening the front baffle of the cabinets makes sense but doing it right would mean removing the old one or using screws every 6 inches. You can not place clamps over such a large structure. My approach to the problem would be to make entirely new enclosures. I would remove the ports, decrease the volume depending on the math and cross to subwoofers at 80 to 100 Hz. That would be killer! 

@unreceivedogma , Boy are you making like complicated.

The free air resonance of a driver is determined mostly by the mass of the construct (cone, suspension, voice coil and former) and the stiffness of the entire suspension. I doubt you will hear any difference between the drivers. Try them both. 

As for the enclosures, each surface has a resonance frequency. Every enclosure is a symphony of resonances. If you stiffen the walls by doubling up on the plywood you will raise the frequency of those resonances. There is no way to know if that will be better or worse than what you have now. Certainly a very stiff, heavy baffle (the surface to which the woofer is attached) is beneficial. It will limit the transfer of energy to the rest of the enclosure. I think I would leave everything else alone. 

If you really want to make them sing get another subwoofer and a two way crossover. Cross at 100 Hz or even higher. The woofers in that speaker carry a significant portion of the midrange which is being Doppler distorted by bass below 100 Hz. Taking those frequencies away from the Altecs will clean them up  noticeably, very noticeably at higher volumes. 

@unreceivedogma , 1+ @mahgister  Subwoofer drivers have a long list of characteristics that defines the size and type of enclosure. Both of your drivers might be usable in sealed enclosures and ported enclosures or only in one or the other. Read this https://en.wikipedia.org/wiki/Thiele/Small_parameters.

Having said all this and just having finished the construction of a new set of subwoofers, you can pick a driver suited to small sealed enclosures with as low an Fs as possible and with a lot of power and digital signal processing make it do whatever you want. A large B X L product ( above 25 T.m) helps a lot. You can push a sealed enclosure lower than a ported one. The ported one will go lower before rolling off but then drops off like a cliff. The sealed sub will start rolling off earlier but will continue to have useful output down lower.  This is a very cursory explanation of a complicated design process. I hate math.