The thing is I do not see the adapter for the 6300 / HC6 anymore on Flex's site.
My Heil Proset came with trs on the phones and ts on the mic connector both 1/8" plugs. This morning after reading someone else had success with no adapter I plugged the phones trs into the phones jack on the 6400m and the monaural 1/8" mic plug into the mic 1/8" jack on the back of the 6400M.
My settings are Bias Off
Mic gain set at 54 currently.
Worked like a champ no adapter even needed!
WARNING THOUGH: Do not ever turn on Mic Bias without an adapter since basically with this setup the Ring and Sleeve are connected together.
YMMV - Another adapter cord not used and tossed in the drawer since I changed from the HM12 Mic and purchased cord :-)