ADC is HD only, the mix cards don't have enough power to handle it.
I believe it was launched with 6.4, but the HD and mix versions of 6.4 are pretty different. 6.4 for mix is really a one off. Like the odd versions you get when new LE hardware comes out.
When my mBox2 came, I was on 6.8.* and that was mBox2 only. 6.4 mix is the same vibe. I also have a feeling it's now unsupported. It's been long enough.
At this point in time, I'd say don't buy a mix system. An HD1 and a Mac Pro is bags of power nowadays, if you can afford it. If not, an LE rig and a modernish mac will probably sound better, have more power, etc.