My first pair of homebrew speakers used B-139s in a transmission line. I built them in 1979. Later I used B-139s in sealed, vented, aperiodic, and isobarik enclosures. I liked the driver a lot, but its x-max was fairly modest by today's standards as I recall.

Duke

Note the shape of the Vifa C13 is very similar to the old Kef B110

I've link to a prelim sketch. The reason for the height of the tweeter is that these are aimed as being my main HT speakers, and 1050cm is the mid height of my screen.
(the second hand nature of the drivers will make them near impossible to sell, so I may as well design them for a specific duty for myself.)

This is a prelim sketch only, so if there are any comments that might help,... thanks

I have also linked to a speculative 4-way series x-o layout which might interest some of you.

http://users.tpg.com.au/users/gradds/Revised%204-way.jpg

http://users.tpg.com.au/users/gradds/3+1%20series%20x-o.jpg

I have here in my hot little hands a paper on transmission lines, with the "textbook example" being a TL spkr built with a B139 and a Celestion tweeter. It has a good drawing of the cabinet, measurements, all that.

Click my name above to email me, and I'll send you this pdf.