I haven't heard the T9x, but had Rel Storm III and replaced it with two F12. Way better and cheaper. Eventually (about 6 years later) went with four F12. This gave a way more even response, plus more body.
I would go with one F12 and see if later you want add another. 2 is generally better than 1, 3 better than 2, 4 better than 3 (Unless you have total freedom in layout - then 3 is probably enough).