i want to purchase one of these 2 headphones. they both seem very good but the hd 518 has better rating at headroom. i am wondering which one is better when it comes to sound quality and comfort. the hd 558 in find a little pricey so i want to know if the difference in sound quality between the two is huge or not. which one is better?

I'm interested in knowing this too

What a coincidence, I was just about to start a similar thread, about the 518s and 598s.

Just ordered 518s so I'll let you know about those hopefully on Wednesday of next week. I'm sad to find that both the 518s and 558s come with 6.3 mm jacks rather than 3.5 mm. They both come with adapters, but I've heard they are bit bulky. If any of you do order either, order from BestBuy.com There is a significant price difference between them and Sennheiser's website. If anyone gets the 558s post back so I can know if I made the right decision with the 518s. Thanks!

i have the 558 and I like it. The 518 probably has a better rating because of the performance to price ratio, but I doubt it will outperform the high model (just as the 598 > 558).
 
The 558 can be had for $120 so if you are paying anywhere close to full price for the 518 (100) you would be better off getting the 558. If you can find the 518 for less however...

thx, i think i will go and get the 558. But it would be nice to know what jebwatson thinks about the 518s.

You can do the black rubber tape removal modification to the 558s to increase the bass.
Can you do the same to the 518s?

Hey guys! I have the 518 and the sound it produces is almost basically the same as the 558 and 598. Refer to the link below showing their frequency and wave responses.
 
http://www.head-fi.org/t/575369/sennheiser-hd-598-558-518-which-one-do-you-recommend-me
 
Listening to all 3 though, which I had the luck of doing, I would say the 518 has the most bass presence while the 598 has the most forward highs and mids. The 558 is obviously the middle ground or the more balanced of the three. Overall I'm very pleased with my 518 paired with a fiio E11 and ipod 5.5 rockboxed. 
 
Anyway, here are my impressions of the 518:
 
http://www.head-fi.org/products/sennheiser-hd-518/reviews/5370
 
Lastly, in terms of comfort, I've been able to listen to my 518 for 8+ hours without having to ever fidget with the ear cups or the headband. The ambient temp in my room is between 25-27*C so that's pretty good in my book. The main difference though between the 518 and the 558 in terms of the ear cups would be the material used. The 558 has the awesome velour that SENNS are known for while the 518 has a velour type version which has more grain and has a square edge compared to the 558.
 
Hope this helps!

@purpleangel,
 
 
I've opened my 518 to see if the black tape /foam mod can be done and it doesn't have it (the tape / foam that is). In any case, it doesn't need the mod because the 518 actually has more bass presence than the 558.
 
As for increasing the bass, pairing it with an AMP definitely made my 518 sound richer and fuller with more ooomph in the base. I compared the fiio E7 and E11 and opted for the E11 since it made the 518 better sounding with my setup.