This is the most obvious mod to anyone who has modded headphones before, but the implications are interesting. Basically, these headphones have the same drivers as the HD598, and the frequency response is VERY similar. The biggest difference that sticks out is the big mid bass hump in the 518's. I'm not a fan of it. However, simply putting cotton between the driver and grille will flatten that hump. The more you dampen it the more it flattens out. And most importantly: The sub bass doesn't suffer! I was doing measurements with my EARS, opening and stuffing etc, and found a great combination of cotton and foam that really flattened the mid bass and midrange. It looks like someone used an EQ to do a very wide Q cut there. I stuffed it so much I had to hold it squeezed shut while screwing it back together, because the foam would push the baffle off of the cup if I didn't. This brings up an interesting point about perceived soundstage... Have you ever noticed how headphones that sound really wide also have a boosted low mids/mid bass range? Well, that's because that's where we're the most sensitive to "width"! I've noticed this when mixing music too. It's a very interesting observation about human hearing. Anyway, I can't say I notice any differences, as I'm not a big "soundstage guy". But this is basically the opposite of the HD515 mod where people tear out some foam.