The HD621 will not work with ARC nor eARC - looks as though the Marmitek might well do the trick, however, I have no experience using a Marmitek. If you can extract to PCM carried on HD blank video then feed to the HD621, the HD621 will take it from there.

808v6, M60, MC200, HD621, Lumagen XE, 504, SB Touch, 563, 201, 204, 208, MPMAX 221, M80, Explorer, Explorer2.