There is no difference between the old Turbo Mod/ Two Thirds Kilowatt Amps as far as performance.
The Transistors/Boards are the same as the old 667V.
I have had both, same same.
Now, having said that, I understand that T/S is now using a MOSFET as a driver in place of the 2290 and I have not seen how these models perform.
If you have a radio with dual finals you are better off with the 500, same final output transistors as the 667 and a dual final radio will drive the 500 just fine.
IMHO the 667 amps require extra effort/create more heat/draw more amps for such a small increase in output is not worth it unless you have a radio that struggles to make 3 or 4 watts.
73
Jeff