I'm not sure how well or not well the bass module is designed. Yes, I do believe is is partially stuffed. Not sure about the 10, but my father has the AM-15 package and it uses a large port rather than a trasmission line and there is stuffing in that one for sure. What I can say about the system is it appears to only lack in the top end giving the system a fairly warm sound overall. There does not seem to be a midrange "gap" as one would expect, however there is a bit of bloat in the lower regions, likely due to the tuning of the bass module - possibly somewhere around 40-50Hz (could be room induced as well, I never took measurements). I am sure the bass module is designed the way it is because it performs well within the midbass range, and even dominates the overall sound of the package, because their target audience is people who have not heard a well built system with a good sub.
One of my father's channels on his crossover died, verified by swapping channels; I am about to take the module apart to see if I can repair it. Will take pics during this...
I auditioned the flagship Bose system in June, on trip to San Francisco. I was at a loose end in a shopping mall while my wife was shopping. I went into a Bose Store and had their Home Theater demo. The small speakers were placed in large boxes and made to look like big speakers. I was not fooled. I agree the HF was deficient, but that's better than too much. Speech clarity was adequate without being spectacular. I thought the bass more than over warm and frankly boomey. I was always very aware of the "bass module." Part way through the demo the true size of the speakers were revealed
At the end of the demo I gave my impressions to the young salesman who was crestfallen. I think he thought he was selling the finest system on Earth.
The problem remains however on how to advise Paulman65. He obviously is not happy with it. May be he has come to the wrong conclusion about what the problem is? Anyhow this system has been a significant expenditure for him. He has asked, "What is wrong with the Bose." Now that puts us on the spot doesn't it?
I really feel the heart of the problem, is the integration of the "bass module" and the cubes. There is no getting away from the fact that if the line is stuffed, unstuffed or partially stuffed, it is not going to be a solution to the problem. I bet Bose have done enough tests with inexperienced listeners, they know which solutions gives rise to the least complaints.
According to your most useful information, Paulman65 is stuck with this unsatisfactory cube/module integration come what may.
Further trying to integrate a sub with that type of system sounds like a nightmare. Unless he has a lot of luck, the odds are against him being happy.
There are just too many issues involving phase, time and group delay, that it would be a real "hat trick." to get an acceptable result. There are so many variables here that results are totally unpredictable. There is more than an outside chance adding a sub will make things worse. I hate to see him throw good money after bad.
My personal advice is that Paulman65, enjoys his system for the reasons he bought it, while he builds up funds to start again, and sell the Bose for the best price he can get.