First, I need to say about my equipment, I'm not a Hifi guy, at least not for now, so I don't have so many experience and equipments. I have a Turtle Beach XP7 incl the ACU (It can be considered as a soundcard), and a Creative Soundblaster SBX external soundcard, model SB1095. And XP7 feels great, good bass and good sound quality with both gaming and musics.
 
I listen musics, play video games, and watch movies, all kinds of stuff.
 
because my XP7's cable is somehow broken, it seems that it's a commen problem, so I bought a Sennheiser 558. Actually at first the soundquality feels not that good, and after about a few hours it got better, and now, it feels not bad, The high frequenz part sounds very bright, that's great, and the soundstage is also good.
but there is something wrong with the mids and bass. I don't know how to express it, the sounds feels muffled when I'm hearing some of the new age musics.
My chain: lossless music -> foobar2000 -> XP7 ACU -> HD558
I tried with the Creative SB, and it feels the same.
 
I searched googled a little, and I don't know if I should do form mod or add an amp or something. It's still not being used more than 50 h, so maybe it will be better after that, but still I need some suggestions, should I do form mod? or add an Amp for it? or simply do some EQ correction? and how to do it?

The 558 didn't change much with burn in for me, and I find them pretty clear and natural. Try a different source I guess, or maybe it's just a completely different sound signature than what you're used to, the 558 have somewhat thick lower mids so they might be perceived as not as clear as thinner sounding sets. They do have a big impedance spike in the mid bass so this could be it, for best results a source with low impedance retains their natural character and soundstaging improves. It sounds great from my iPad for example without a need for amp

Yep, it sounds the same on my ipad air 1. I have a HTC One M8 and Ipad Air 1, I tried them, and they have the same problem.
 
Maybe you are right, It's a completely different sound signature than I used to.
 
but should I get a Amp? or can I correct this by EQ it?

And I have another question, it sounds weird, but..
 
when I play pokemon on 3ds, I tried use this headset, apart from the audio quality, the soundstage feels great, but when I plug this into my computer or my TB XP7 ACU or my ipad, the soundstage is not that good. Is this because of the music?