I use Potplayer with LavFilter set up to LPCM, exactly the same as for the HD621. I will try to feed it with bitstream signal to see how it behaves but I'm guessing it doesn't convert bitstream to LPCM.
It's indeed "castrating" the signal delivered to the HD621, that's probably the reason my unit doesn't have drop outs anymore.

HTPC, HD621, 861v6, Pathos Cinema X, Kinoton Cinema 200, JVC X7900