Bun Bo Hue (Vietnamese Soup)???
What do you mean by authentic? Very spicy & with a pigfoot? I think Pho Tu Do does a good one.
The best I've had was in Montreal, at Pho Bang NY, which is apparently related to our Pho Bang. They only serve it on weekends there. I don't know if Pho Bang in Chinatown makes it.
I've been to Hue, and I don't remember the BBH being significantly different from what I've had here.