I hope this would be interesting to all fans of this headphone, as well as people looking for great bang for buck in closed on-ear.
I got a used pair of 518DJs and although I generally liked the sound, there were just a couple things that bothered me. The mid-upper bass was strongly emphasized, giving it a bit of a wooly impression, and overall the tonality was rather dark. Removing the foam, as you know, lets through a bit more of those highs, and that was the first thing I did. It still wasn't quite enough for me, so I decided to pop these open and look inside.
The mods are detailed in this imgur thread here:
http://imgur.com/a/FExeh
I haven't done the Blu Tac mod - I suspect this can help "clean up" the bass a bit more, to make it a bit faster and tone down the cup resonance a bit. In terms of overall tonal balance though, I'm very happy with where this is at now. I think removing the rubber coupling piece behind the driver was probably the most important for getting rid of the upper bass hump - it was essentially sending vibrations to the rear wall of the housing, in an attempt to create that "DJ" sound with overly emphasized bass. The rubber also seemed to reduce the dynamic range of this driver somewhat. I'll have to A/B with a stock version to be sure. The bass port mods were also key here to tuning the overall signature.
The end result is that these are no longer a bassy headphone at all. You will be surprised at how neutral they sound now. It's like the AKG engineers took a relatively neutral driver unit, and tried their damndest to build a bassy DJ headphone out of it. Ripping out all the things they did to achieve the bassy tuning is the essense of this mod.
Right now they're tonally more accurate than Sennheiser Momentum 1 over-ear, probably more neutral than HD25-1, has more clarity and detail than Creative Aurvana Live! 1. Although the mods improved soundatage, it's still not as spacious sounding even compared to Momentum, but I'm willing to accept that compromise for this price.