Bass on HD555's suddenly sounds muffled?

I got a pair of HD555s around a year ago. They sounded great, no serious problems. I didn't use them for a couple of months, and now upon using again they sound kinda muffled. I think its a problem with the bass, as if I turn bass booster on in itunes the problem gets worse while if I turn it off its less noticeable (but you just don't get any bass so while listenable its really just as bad)

I really don't recall having these problems previously -- they are noticable enough that sounds sound funny so something has most certainly changed with them. Some quick internet searches indicate similar sounding problems with new headphones needing to get burned in, but these aren't new I just was using other headphones for a few months because I was too lazy to pull my HD555s out for some odd reason. The cable does like to twist up on itself and get a little tangled - but that happened before too and there are not any sharp kinks, just twists and loops and stuff. 

Any suggestions on how to fix? I really liked the headphones before this but given this and problems with previous pairs of headphones I'm beginning to doubt value in buying swanky headphones if they are just going to crap out after a year.

I have been using my earphones (Etymotic MC5s) in the meantime, so it isn't a driver/ear damage issue. I don't recall thinking "Wow the bass on these things is so much better" when I first started using them though. (and it comparision, the 555s certainly seem to have more bass so maybe I just wasn't used to it after a while)

Then seem to be sounding better now. Still feel a little off, but that might be placebo effect.