Do you have a MIX set up?
Legacy conector AFAIK is for conecting black old school (888/24, etc) converters, not a host system, but I can be wrong.
Name the interface you have on HDX.
If you just love those old plugs I would just use the latest version (the VST one) with an M-Audio AP 24/96 and 2 SPDIF cables. And Just install DieHard Instant Cubase DAW. I think they are included, but I could also be wrong or maybe you just prefer the old TDM algorithm.
With a Digi 001 card you have ADAT in+ Adat out, that could be more productive.
If you like to run those plugs in a TDM environment + Protools TDMprepare to learn WTF is a MIX, a MIX+, a TDM, etc. Only worth trying if you already have the hardware.
If the HDX has AES/EBU then a black 888/24 + a Mix Core may serve you, but again, I am expert in LE setups, and HD since PT 8.