Try playing a nature sound tape or CD that reportedly helps reduce subvocalisation. If you haven't got nature sounds try Mozart. You find that you subvocalise less and less as you progress with the process so just keep going. As your speed increases subvocalising will reduce.
So keep pushing your pace with superreading and dipping... you'll find you don't have time to subvocalise and what you... doesn't matter you're getting your reading and comprehension down a lot faster that many others 
Alex
