The drivers are now injection molded instead of vacuum formed. Also they are a slightly different material which they call metalisized. The specs are very similar with a little more bass extension. The new drivers are supposed to be stiffer and lighter so they won't distort as much.
I just bought a pair of 6Ts and so the only difference that matters to me is the new line made mine a heck of a deal! let me know if anyone knows any other differences.