What you describe is fine. Over the course of last years I had a number of people who report identical things to what you do. Even if you change volume of back chamber and move resonance frequency then sound will change but it most likely will not produce proper sound. BTW, I would pay your attention that you did not mention what driver your horn use and how sound change when you move Fs 10Hz up.
Considering the amount of homework you did and own thoughts you put into asking this question I will collaborate on it but be advised that the subject you post is incorrect as what you complain has nothing to do with resonance frequency. In my view the Bruce Edgar’s midbass horn is wrongly built and designed, at least in the version that you have.
First thing first: Bruce Edgar is the most prominent and the most knowledgeable horn designer and builder alive today, in fact he has been holding this title in my view during many-many years. In addition he does hear the things very well, has cultural and cognitive capacity to interpret properly what he hears - a very seldom quality among individuals who are trying to make acoustic systems. Furthermore, I consider Bruce as my friend, consult with him sometimes and even during my midbass project we spoke a few times and we collaborated on some issues that I had. So, it would be greatly inappropriate for me to criticize the Bruce’s 80Hz horn.
However, one of the keystones of this site is honesty and respect to the actual sonic results. From this perspective I do not feel that criticizing Bruce’s 80Hz horn I violate anything. If Bruce’s, your, mine and anybody else objective to advance horn sound instead of patronizing horn people then criticizing Bruce’s 80Hz horn will be a great respectful service to what Bruce taught all of us for years.
I do not like Bruce’s 80Hz horn. I heard it only once, I did not like it then but I even more do not like the design idea. 8-10 year back Bruce furnished his Titans with curved 80Hz horn, even that curved 80Hz horn had some problem it in my view was much more interestingly sounding horn. Then Bruce desired to bless the Morons with a straight horn – I think it was a disastrous move: the horn is too short with too large throat, and at least with drivers the Bruce use they shall sound in the way how you descrive.
The need to note that I used the word “Morons” in the last sentence very deliberately. There is Bruce and there are his customers. It was my vocal and persistent position over the years that Bruce Edgar’s customers in dominating majority are incredible idiots. Since I entered the horn field in 1999 I was dealing with them many times and I can testify that 95% of them are remarkable retards. I can publicize many events when I and the some “specific” Edgarhorns owners where involved and it will be absolutely self-evident how much Morons they are, the last even was 3 months back with a retard posted at AA under a moniker “Dan_ed”. The problem however is not only that I know it but that Bruce Edgar intelligent to know it as well. So, what Bruce doe it mitigate the design and implementation efforts in accordance with the level personal stupidity and sonic ignorance of his customers. I would do the very same in I were “in business”. So the short 80Hz straight horn was sort of Bruce’s Hoi Polloi horn for the Morons out there, sort of Bruce’s Volkswagen horn (I original meaning of Volkswagen).
So, I do think that you will be able to fix anything in the Edgar 80Hz straight horn. You can change driver, move Fs up and down, beef the enclosure with sand but it will be all half ass solutions. I personally did not play with this horn but in my view, looking at the short straight horn with huge throat I do not see a horn but rather I see a direct radiator shooting through a pipe. Many Edgarhorn users do “upgrade” of their Edgarhorn replacing the old curved Bruce’s horn to the new straight one, so you might pick up the old one that will be much better. Alternatively fine some kind of idiots who would buy this appendix horn and ask Bruce to make to you a properly horn if you will be able to accommodate it in your system. (I personally believe that you will not as I feel that 80Hz horn is too large to be located under MF channel). However, talking with Bruce you need to specify very precisely what you want. Bruce is not a fool and he very much sees what people are trying to accomplish and he will be able to support any level of your demands. However you need to truly have the evolve demands and very precisely nailed down the requirements for your project in order to make Bruce to do what you want. You need to understand that Bruce is accustom to deal with his typical customers – the fucking audio retards that will swallow anything as long it was chip and “recommended”.
Sorry, my reply was not in the direction you would like it to be. If you would like to play with Fs, then set it to 80Hz and it shall be fine, most likely Bruce has do it already. Decrease loading of your tube if you use tube amplification for this channel. More idle tube will lose gain n power but will give you more control over the diaphragm. Rgs, The Cat
"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche