UH400. UH400a, UH400TM v6.4

Lowered corner frequency of subsonic filter, to preserve low frequency performance required by UH400TM model. Other models roll off low frequencies in analog, such that this change to the digital filter has virtually no effect.

Note: It was not necessary to backport this feature to the 3.x or 5.x branches, because no UH400TM has ever used the old PLL.