Reasons

iMF !!!!!

"One thing I learned was, respect for those who had mastered the art of transmission line design."

Art is the right word to describe those older designs. By intuitive feel and a lot of trial and error a few people could build very good TLs. But there were more failures than successes.

Now with the computer design tools available, a TL can be designed on the computer and perform as predicted. No more myths or snake oil, an understanding of how a TL works and what trade-offs are available is easily accessible. The accuracy of TL design is on par with the bass reflex and sealed box enclosure designs. Measured results correlate with the predicted response. The calculations are more complicated but the rewards/payback are greater.

Bob Brines, Jim Salk, and Dennis Murphy to name a few have all used my software (Paul Kittinger did the actual TL design work for the later two) to design the TL portions of their speakers. Bob Brines was a very early supporter of the software and is a very talented designer, his understanding of TLs is excellent.

I think there is some adjustment potential with a TL design. Adding internal obstructions and/or a constriction at the open end can always be used to change the tuning frequency and response. Also changes to the stuffing density and location is another tweak that can be used to change the TL's response.  

Are you saying we could build a TL speaker enclosure, to accepted design, ....and then play around with the exit size to see if it enhances the sound?  (along with changing the stuffing and its placement)  This sounds like 'fun' to me!