a)      Cut off frequency of mouth
b)      Diameter or surface of throat
c)       Profile and size type
d)      Crossovering used
e)      Driver

Without naming those characteristics the horn is described and to make a clear picture of what one does or trying to do is very difficult

2)      A proper LF solution will not give you an opportunity to high pass your bass horns and unload the bottom – it needs to be done anyway. In some case what the Fs is relatively high, close to the horn rate it might be arguable. In you care you use 100Hz horn (presumably) and a driver with resonant frequency of 50Hz (presumably). You will be able to raise the resonant frequency for 15-20Hz via your back chamber but it still will be a good 30Hz under your horn rate. I do feel that in this case the unloading of bass is necessary. If not you will not get more bass – horn will cut it off anyway but you will get worse bass in the region that the horn will pass

3)      If you (your wife) like a white colors for the horn then why do not finish the MF and HF channels in white? I think it does make send to keep the colors of the entire acoustic system consistent.

4)      The time alight you were trying to set is fine but I see that tweeters’ axis are not firmly fixed. You need to make absolutely sure that axis of your MF and HF channels are parallel and will not be altered by change. Any minute angling of those drivers will toss the time alignment away. You can ease test it and to see that a very minor change of angle of HF driver will offset your efforts for time alignment. I see that you have some kind of mounting plate on your tweeter. Use it and bolt the tweeter to the frame.

5)      How many DB you need to attenuate your MF channel in respect to you upperbass channel?

6)      You might consider driving your upperbass from a separate amp if the attenuation above is too high. Get for teat some kind of 50W SS amp and see what will happen. Be as that if you use high-damping SS amp then the influence of back chamber will be much more negligible. The result will greatly depends on the driver you use

7)      The 12” driver, what is the size of the throat? I guess it is 7” by 7”?

8)      In the response you posted I think you use a tweeter as well. Anyhow, I do not think that you will be listening on axis but most like at some angle. I would encourage you to discard and never use the on axis response as it has absolutely no practical meaning. The point I am trying to make that in all measurement it is critical to have a very direct relation between you hear and what you measures. If you do not use your horns axis then do not measure them on axis

9)      I would say that your MF-HF channels are a bit “in the wall”. I would like them to be more extended into the room if you have space. When MF horn will stop “reading  the walls” it will give you very pleasant auditable space behind the speakers.

10)   If you would like then you might consider adding one more channel – the lower MF channel, or something that I call a Fundamentals Channels.  In your case you might do it in very interesting configuration. Take a look. You remove your MF from the backs of the upperbass and point you upperbass channels to the walls.  Loading then die to the walls will give you an extra let say 10-15Hz of bass + you will have more mature bass. Then you move your MF island for 3-5 feet ahead and place it atop of a new Fundamentals Channel.  This new Channel might be some kind of Altec 288 driver in 180Hz hyperbolic rectangular horns. The Fundamentals will be running from 350Hz-450Hz to 800Hz-1000Hz and the rectangularity of the horn will not affect sound badly.

11)   In your current configuration your MF island does not look like aligned with your upperbass. The upperbass cone is somewhere at the beginning of the back chamber but in S2 driver the cone is in very back. So, I presume that you have a good foot that you might move your MF island forward toward to the mouth of your upperbass. In addition to the alignment it will make the system more compact and more proportional.

12)   The 3 sections of  30, 40 & 50 cm long – did you chose them by some kind of resonant modes principles or it was just an arbitrary decision?

13)   Bass section – this is difficult.  I also would like to hear the tapped horns but I did not yet. For a time being you might consider getting some kind of commercial high price subwoofer as a temp solution. Look for sealed only active devises of high price. Get 2 of them used, use then for as long as you need, and then sell them. You will not lose a lot of money but you have a good buffer for a time being. Among the commercial subs you might look at your British Rel Subwoofers. The do not shake room during those stupid films but they are relatively acceptable for music, this top of the line models.

14)   What was the total cost of your upperbass project?

15)   How do you joins the 3 sections – juts glue and why you decided do not brace them in the joints?