Free air resonance

@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 

I have no dog in this, but I am curious why you have the Velodyne set up so high?  I have never had a subwoofer set past 100hz with like a 12db roll in.  

Oh and BTW, you have three of the smartest and nicest people helping you here.

@curiousjim 
— I am not aware that I have any control over the Velodyne crossover setting. It comes at 85HZ. I don't know where you get over 100. The Velodyne frequency response is 18-85hz. The resonant frequency is <4hz

— Yes they are great! This is a fun conversation.

@mijostyn @erik_squires 

— Altec's 620 cabinet is 18" x 26" x 40", for 9 cubic ft. Mine is 18 x 26 x 38, with a 2" base

— The diameter of the driver frame is 15.2", so 18" leaves little room. Otherwise, I am trying to visualize your cabinet design. I guess I would have to draw it out first, but it seems quite ambitious: not easy to execute. 

— My latest thinking is to start at the beginning. I've had the 604Cs for 40 years, the D for 25 years. They haven't been re-coned in almost 10 years. They are all over 60 years old. It might be wise to send them to Great Plains to get them re-magnetized and refurbished. Then fool with the cabinet.

I forgot to mention this but honestly DIYaudio is a much better place for speaker building questions than Audiogon.  Lots of builders with first hand experience into all sorts of designs. 

Best of luck,

Erik

@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.