Well Shady, I have good news. This speaker is capable of very significant improvement. As I suspected the pipe volume is too small and the taper not adequate.
These speakers model very well in a TL. You can get and F3 of 50 Hz, in a TL when Fs is half an is about a quarter octave higher. 6db point is 40 Hz and it still has useful output to the 30 Hz region being about 18 db down. The pipe works very well with driver using only 1 mm of it 4 mm excursion at full power. The model shows how well a TL properly aligned can control cone excursion.
The pipe volume Vp is 50% undersized. Now the designers may well not have been aware that Vp has to be calculated. This was realized only 19 years ago. Until then pipe length was though to be all important. The realization the Vp had to account for driver Vas was a huge breakthrough and explained why design of TLs had been so hit and miss with a lot of suboptimal designs. The taper for these speakers needs to be 4/1. To get this and the required volume would mean making the speaker a total of 14.25" deep if 3/4" MDF was used, which it should. The turn should be properly turned over with the correct internals at the top. The port should be 3" high.
Here are some screen shots of the model.
FR, Impedance, cone excursion and group delay.
Model showing only 1 mm cone excursion.
response limited to 500 Hz
The model specs.
This design change would of course increase cost, as there would be more material and shipping and packing costs. All these multiply up. However this speaker would be a bargain at three times the cost. The acoustic response of those drivers is excellent. They only cost only $12 a piece!
This is a tantalizing design and if produced to the correct dimensions could be not only outstanding value but among one of the best and most listenable speakers at any price. It shows you can get very respectable bass performance from 4" drivers. I can get similar good bass to 40 Hz with my vintage 4" JW drivers. So I have known for 60 years at least that you can get good bass, and I mean good high quality bass from 4" drivers.
I would strongly advise Dayton the market an higher end version of this speaker. It would likely be a very good seller.
If any member wants to build this speaker I would strongly encourage them to do so.