I did discover that one feature seems to be lacking from the Phoenix, and that's CAST-output. I would figure that could be a useful feature if you would use it as a preamp.
Indeed, Kingwa confirmed that the C3 should make a very good HP amp, though he hasn't tried it used in that way. (The C3 should have no problem driving headphones with 50 ohms impedance or higher, like 250 ohms. For lower impedance, like 16 ohms, one should change a module in the C3.)
If in the next few weeks I can modify my DT-880 to have XLR outputs (I have no DIY experience), then I will use the C3 as a HP amp, and report back!