[zM4RTIN]

I've had the Superlux HD668B for about a week now. I've had it on some books every night + about 6 hours in the day, but to no avail. What can I do to make them less tight?

 

[Notus]

Superlux HD668b has the same type of headband design as the Akg K240 models but i am not sure about the materials used in the superlux if they are as durable as the Akg K240 headband. If they are you can try to bend them slightly "backwards", this is pretty much the only way to relief the clamp on them. Be careful not to over bend them, little at a time. Try them on your head to find the right amount of clamp. Remember bass can suffer from too light clamp.
 
You can try to take the drivers into your hands, put your thumb on the headband rails near the cup so it wont start to bend in that part and snap, then you slowly start to bend the headband "straight". Do this at your own risk since i don't know about the material on the Superlux.

 

[zM4RTIN]

It's plastic. Can I really bend plastic like that?

 

[TheEndGame]

I used a pillow and left it for a few days.

 

[Notus]

There are a lot of different types of plastic... Like i say'd i am not sure how durable the headband is compared to the Akg K240 models.
The K240 can take some serious bending and it wont break. You can always try the book / pillow / box stretching routine for extended periods but remember to check the headphones from time to time so they wont over stretch for your head. If you feel like its not working, increase the amount of books or what ever you use to stretch them.