I think the original drivers in the 415 cab were Sunn Magna 1516's.
I have absolutely no idea what the TS parameters were for those drivers, but that's probably a moot point because the folks who designed the 415 cab didn't either; that cab was designed before TS parameters existed.
That's a pretty small box for four 15" woofers, but I think the key to finding proper replacements is that they were
16 ohm drivers, and that can limit your choices for current production speakers.
The WGS BG15C
http://wgs4.com/content/15-BG sounds pretty good in every box I've heard it in, but I've never heard four of them in such a tight box. They
are available in 16 ohm though, and are currently my "go to" driver for reflex 1 and 215 cabs. I love my JBL's but they are fairly low powered and getting scarcer and more expensive all of the time, so I'm less prone to beat on them. The BG15C's sound great, handle 300 watts each, are readily available, and are relatively inexpensive.
Another idea would be to call Eminence and see what they can suggest. Have the box measurements and configuration handy, I think you will find them quite helpful as well.
I don't believe they offer a 16 ohm woofer, however that shouldn't be a major issue to wire around.
Good Luck!