I just got a pair after reading this thread. Out of box they sound way worse than the HA-S500, but way comfier. I think the most appropriate term for their out of box sound is veiled. I'm a bit skeptical of burn in, but I've been playing these above listening levels as much as I can and am not noticing much difference, but will keep at it since I'm stuck with them. Biggest difference I've found is replacing stock pads, anything with more breathability does these headphones a big service.
The earcups are shallow, so I don't see much space to mod but will try at least to throw some dampening materials in there (fiberglass). Listening to the driver without the earcup you hear a huge difference, very good sounding drivers.