This is the part that I would have to disagree. The idea that different speakers in the same cab make a huge difference.I agree that the difference between open and closed back will be huge, but
Maybe I'm misunderstanding you, but the variations between different speakers in the same cab can be huge, it can change the sound of an amp in a major way.
What do you mean 'if done properly' ?
My whole point, in my opinion, is that this speaker difference is not nearly as much as you can hear between the same speaker in different types of cabinets.
I base my opinion with these observations...
I know personally what the difference it makes to a speaker placed in a closed back, open back, combo amp,
and above all my newly aquired Traynor Dark Horse cabinet which makes any speaker I have put in there sound so much better then it did in any other cabinet ( and this difference isn't close)
I have also seen plenty of youtube videos of speaker shootouts in the same cabinet. There are hundreds of such examples on you tube.
In most cases, the difference is so close it is almost impossible to tell which speaker is being used if you close your eyes.
HAve I covered every speaker combination possible....not even close...
All I can speak to is my personal observations and can totally understand
that others have different experiences and come up with different conclusions.