Free air resonance

@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. 

@unreceivedogma 

Sorry, I must  have misread.  I thought you said you had the Velodyne crossed at 2k!

All the best.