When I got them they were overly bright and after 50 hours of burn-in with a pink noise + sweeps track (supposedly by JBL) that I found on YouTube they got even brighter - piercing and fatiguing even. I'm thinking either the sound used for burning them in matters _very_ much, or I got a bad pair that will simply never reach that mythical neutrality everyone's been gushing about around here. At any rate, I'll use bass-heavy music for the next 50 hours and see what happens. Not happy about my purchase just yet, cheap as they were (got them for about $24).

That's a good price.  I've discovered that I can't use them for long at high volumes, but I'm not a kid anymore so I have to protect my hearing.  I'm a retired musician and I think being away from "loud/ive" music, I'm finally getting a bit of perspective on what bugs non-musicians about musicians preferring "Conductor's podium" dynamics as opposed to "Row H".  For the first time in my life I've experienced listener fatigue.  All that said I still like these cans, but I don't run them "hot".  Also, remember to use at least a couple of layers of something to take the edge off the highs. I used several sets of of those pads from the other earpad manufacturers.  Lastly, I've happened across a headphone that I like almost as well and that's a couple of MCM electronics Stellar Labs phones: The first is a Red and Black knockoff of the AKG's that the Superlux folks have taken a shot at.  But there is a SECOND (new) 'phone that I have great curiosity about and that's this phone: (Stellar Labs #35-5985-model HC-5985) It looks promising and is presently discounted to about $40.00.  Not as inexpensive and the shipping's extra.  But eventually it'll go on sale and then it might be something to experiment with.  The Red and Black 'phone (HO-5960 MCM Part #35-5960) is less aggressive in the highs and worth the ask.  The Bass might well be a bit better than the Superlux 668's.  When I find out I'll update my review....thx for posting!

Update:  Here's the URL for the headphone I was referring to...it's lacks the brightness of the HHb and has (to my ear) slightly better Bass response.  It's a bit more expensive and is NOT quite as much of an homage to the AKG design.  You might prefer it, you might not.  Don't try anything without knowing the return options.
 
http://www.mcmelectronics.com/product/35-5985

Nah, thanks, I'm not on a mission to get the best headphones below $50, I just casually jumped on the 668Bs because of the staggering difference between the praise they've been getting and the price I found them for. Other than that, I wasn't really looking for new over-ears, as I don't have a major use-case for them. At home I use my speakers for everything, at work it's too noisy for open designs and my attention will be on something else anyway, so no critical listening to be done, on the train it's even more noisy, and on vacation it depends a lot but portability is a primary concern and on that count the Koss KSC75s have been great so far.
 
I'm still going to take the 668Bs as far as I can with the burn-in (highest recommendations I've seen said 400-500 hours), but at 125 hours in I'm already starting to suspect they will never get anywhere near as impressive as the most exuberant reviews made them out to be, because of how much talk I'm seeing about changing pads and adding paper tissue or other materials to tame the highs. I'm thinking it might be useful for reviews to always specify if the sound quality description is based on a stock unit or a modded unit - this would remove a lot of confusion with models like this that don't really sound great out of the box.

The MCM 35-5985 is a better out-of-the box- 'phone than the 668, and the build quality's pretty easily (objectively) ascertained as being better. But my experiences in other life events tells me that "herd mentality" will prevent this other phone from ever catching the 668b.  It's okay.  I don't have any connection to either.  It's just worth checking out for folks with "headphone acquisition-itis".  I caught it after having lurked here for a few years.  It was "contagious" (for me) because of what's going on in the Far East now (industrialization).   I see a similar thing happening there with personal hi-fi that I believe went on in the '70's in Japan with broader market High Fidelity.    There was a peak in quality/price/performance in the late '70's that hasn't been matched since.  The (not all that fabulous at all, from my on the ground FPS experience)  '80's saw audio gear get lighter, cheaper, and far less well made/engineered as prices escalated into orbit owing to the recession of the time. The trend started to right itself in the '90's and then went back off a cliff again after the turn of the Century.  There are a lot of vintage audio sites that speak of this phenomenon as the onset of "BPC".  (Black Plastic Crap....) And it seems to me that this will (eventually) happen in personal audio as well as the market saturates.  The "high end" will always be there, but now seemed like the time to explore personal audio while the "exploring" is good and affordable/desirable.  I hope it lasts longer than the "second golden age" of High Fidelity did (I wasn't around for the first, it happened during the decade before my birth) but at least I got a "few" licks in during this "Golden age" for personal Hi-Fi.  It's been REALLY educational.  Many important steps forward have been achieved and Head-Fi appears to have been to personal audio what "The Absolute Sound" and "Audio" (magazines) were to the broader High Fidelity market in the '70's.  YMMV (Always....)  Best of luck.