So I picked up a pair of 579s because I wanted to replace my 558s, but I didn't want to get up into 300ohm "you must buy an amp to make these work" territory (yet). Curious as I am, I decided to crack them open to see what was inside. Those who know Senny cans know that the 555 and 558 had a "foam mod". Inside the actual ear cup is a strip of foam. The foam bounces the sounds around and is meant to give the headphones a stronger bass response among other tonal changes. Well, my new friends, the 579s have that exact same strip of foam in them. Okay they're not exactly the same. The foam in the 558s were more porous and soft. The foam in the 579s are smoother and stiffer. I would wager a guess that it works to reflect the sound better. Steps to complete the mod: 1. Remove ear pads (they pull off, just like the 558s. In fact, they use the same sized ear cups as the 558s and you can swap them with 558 cups (and vice versa) if you want to). 2. Remove the three screws to pull the driver cover off. 3. Carefully remove the driver/driver cover. 4. Remove the foam. 5. Put everything back where you found it. You can find a video/photos here. It's literally the same process and steps as the 555/558 (which is what I linked just above). The insides just look slightly different. Actually, they look the same. The 579s are really not much different from the 558s. Q: Is the mod reversible? A: Yep, just be really careful removing the foam because it rips easy. Q: Does it void the warranty? A: I'm sure it does, but it's totally reversible and there's no warranty tape or anything to break to get in there. You can reverse the mod and Sennheiser will never know you were there afaik. I'm not saying you should scam anybody, simply that Sennheiser didn't take steps to prevent us from getting in there in the first place so I can't imagine why they'd care as long as you don't do something stupid. Q: How does it change the sound? A: It's not super dramatic or anything, but the changes are there. 1. It removes a bit of the "oomph" on percussive instruments. A good A/B test track is Weezer "Island in the Sun". It has a clean drum part that should depict the differences well. 2. It removes a bit of the harshness of the upper mids. The 579s can get a little weird on vocals in general. Journey "Separate Ways" is a good example. Sometimes his voice just goes off the charts and borders into harshness. The mod tones that upper midrange harshness down a little bit. 3. The lower bass region is a bit more boomy and a bit less punchy. As a result, the bass is a little slower and less attack-y. It's not altogether different from the 558s with the foam mod. The bass is just louder and more clear on the 579s. That may or may not be a good thing. I'm not sure yet. 4. The sound overall is less harsh and less fatiguing. It gives it a warmer, more relaxing sound much like the 558s. The 579s have stronger upper mids and lower bass than the 558s (by a little bit), but otherwise the two sound remarkably similar with the foam mod performed on both. 5. The treble rolls off a little harder. This helps contribute to the more relaxing sound. The treble is still there for sure, but it's a bit less clear and a bit less prominent. We're not talking a huge change here. If you listen to a song on unmodified 579s and modified 579s, there isn't hardly any difference to the overall sound signature. It sounds like what it would feel like to take sandpaper to a piece of wood. 99.9% of the wood is still there, but it's smoother with more rounded edges. That's what it's like. A lot of stuff that was previously harsh, loud, rough, or percussive is smoothed out a little more. If you like the bass punchy and you don't mind the slightly harsh upper mids, then do NOT do this mod. If the harsh mids are killing your brain and you don't mind the bass getting more boomy and slow, then the mod may be a good choice. Anyway, just thought the Senny fans would like to know. I don't know if the foam is in the 599s or the 559s, but it's worth a look (at least in the 559s). They might be in the 569s, but I don't see why. They're closed back, adding foam seems superfluous, bit hey I could be wrong. Q: How do these compare to the 558s? A: The 558s are like 98% what the 579s are. The 579s have SLIGHTLY better lower bass and a stronger upper midrange. The build quality is also way better. However, it's not worth the $150-$200 to upgrade. Keep your 558s and your money lol.