The reason arises from the fact that there are delays caused by the crossover circuits and the fact that delays are caused by the drivers being offset in distance from the front panel (which is a reference point). Actually, the natural roll off and roll up of drivers cause delays too.
At the crossover freq for the mid and tweeter both are sounding. Because of the aforementioned delays the mid and tweeter output at the crossover freq are not in phase. Apparently they are 180 degrees out of phase and this causes a cancellation and a resulting dip in overall response just there. It is easy to see in the acoustic frequency response curve.
The solution is to reverse the polarity of the tweeter which puts it 180 degrees more out of phase overall and this means it is back in phase or 360 degrees relative to the mid. Therefore, no dip.
It never fails to amaze me just how complex the answers to simple questions about audio can be. I read through the responses, and I was prepared to stop reading after looking at the graphs Chris pointed to. Everything from the rule of thumb answers to the logarithmic scales is pertinent here. To have more precise answers, the question needs to be much more complex and specific. Perhaps simple questions are simply rhetoric questions.
How do you ask a question that yields lower subjective response and more measurable and objective response? By asking very specific questions. How about this one taken right from the OP:
I have A/B compared the Chorus and Forte II. Why is the Chorus quite noticeably louder with only a rated 2 db difference in max output than the Forte II?
The most likely and obvious answer to that specific question is that the Chorus is more efficient by 2 db.
Bulbs are easy I bet !! I dont know everything we just rely on info from places like this. I really like this forum !!! Lots of knowledge here.. I think Bob does know his shit when it comes to speakers.. I'm a guy that cant afford to spend 300 bucks on a cap upgrade done by somebody else. I have always figured If i can get the tools and the parts we can fix it just like any other man The Internet rules!! Lots of info!! Only problem I run into is with newer electronics . Most of this old solidstate and tube stuff is really basic. It only becomes a problem when the cost of the parts or tools is greater than the value of the Item The other day dads flat screen died lost the power supply for the backlighting fixed the power supply with spare parts i had from other junk. Then one of the boards that controls the hdmi inputs died told him to junk it!! Cost of that board 300 bucks!! Dont have and cant get info you can get a new tv 42inch for that.. I have been really excited about this vintage audio hobby and the stuff just rules over the new stuff!!! Easy to work on powerfull "ITS AWESOME"!!!
I don't think the tweeter is anything special on even the high-end BWs. I've never heard a Klipsch speaker that bright. But they market their design as "cutting edge" and such. I've heard that to replace the tweeter motor it's only like 75$. Having to replace a tweeter motor on a high-end Vandersteen involves Richard Vandersteen actually building it. It will cost more than 75$ if out of warranty or abused.