Generalizing from the particular is not good form. The data to which you link indicate that (at least that) 509 is not a particularly good tube for that service. But my screen-drive amp shows better than 0.5%THD/1kHz/10W, the tubes act nothing like those curves, and tubelab has independently verified those results. Try something better like 6LF6, paralleled 6JN6, or something else actually made for TV service rather than rebottled tubes sold for audiophiles.