Thanks for the warning zazex! 

try an aftermarket cable, like the hand braided silver plated available on e bay for 40$ or a custom  headphone Mission cable made by Zu Audio....evaporates the sibilance, decongests the sub bass and takes these wonderful phones to another level that is magical and auditory bliss...

drbluenewmexico - your comment is helpful and well taken.  However, Thinksound does not include the cables you mentioned with the On1 headphones, and I reviewed the headphones with the cables as supplied in the retail package.  With those cables, I experienced the significant level of sibilance to which I referred in the review.  
 
(NOTE: Out of curiosity, I did try a couple of high quality aftermarket cables, which was simple to do as the cable connection to the headphones is via a standard mini jack.  And, as you said (not entirely sure yet if this is a placebo effect) - the sibilance was gone.  Remaining was the great sound I'd mentioned in the review.  I suppose there's much more to be said about this, but I'll not do that now.)

You also should/could have them tested for more than 24 hours playing time, they will sound a lot better by then. Going to score the aftermarket cable as well. I will be happy with these for a long time. 

HurricaneBill,
 
I agree with you.  These headphones sounded so good (despite the sibilance) that I decided to keep them.
I've since used them with four other cables, and there is no sibilance with any of them.
 
So credit to thinksound for designing and producing this great set of headphones, and a "debit" to them for
including cables which diminish their SQ quite significantly.
 
And another credit for allowing the use of any 3.5mm to 3.5mm cable.
 
(Aftermarket cables used: Mogami 2893, Audioquest Big Sur, AudioQuest Mini 5, Mogami 2528.)
 
_______________
 
 
Over the next few days, I'll find out how to raise my rating on these to 4 or even 4.5 stars.