Perhaps.  Supposedly Hifiman released a batch that had to be recalled, and it was known for very apparent treble issues-- it had sandy brown colored plastic grill over its driver.

Yes, while the HE400 is slightly bright, the HE500 is very slightly dark. 

My HE400 (one of the first batches) sounds slightly dark to me.

Early revision (rev1 black driver, as well as the earliest white-driver rev2s) were as I recall very dim in the treble. Current-production HE400s are much much brighter in the mid~upper treble. It's such a huge change too, not like LCD2r1 -> r2.

Interesting. My pair is bright in the last octave, but really laid back in the upper-mids and lower treble.

Same here.

My earliest pair (which was replaced as it had cut-out issues, not surprising since it was a January batch) was really rolled off in the treble though, and from all the early 2012 official HE400 reviews, all the reviewers received the early dim treble versions too. There was no tizziness issues with any of those because treble was very little in quantity. Then Hifiman silently retuned the drivers into the present bright version.