Basic history as far as I know it: Beyer introduced a new bowl-shaped earcup assembly in 1990, presumably to replace the older cylinder-shaped cups of the DT770/880/990. First it was four higher-end models: the DT801/901, which were closed, and the DT811/911, open. In 1997 this was simplified into the much better known DT831 (closed) and DT931 (open). In between three lower-end versions came out, the DT331/431/531, of which the last still has a cult following.
I've never heard a DT811, but judging from the reputation of the 831 and 931, it should be detailed and bright, but a bit lacking in bass impact. Cash value is probably around $60-80, though it should perform well up against new phones at that price.