The 5S that I purchased last year had a serial number that began with F2. It worked quite well. My iPhone 6 starts with DN and I just had to send it back due to a button defect. I don't know anything about DX, though.
I had my previous phone replaced (iPhone 5) and the replacement also started with DX.
I don't know whether it was an actual refurbished phone or one of the white box models that they set aside for replacements straight from the main assembly line.
Either way, they're basically brand new all the same and other than the serial number they're the same as the rest.