Hope I'm not re-posting, but apparently Sennheiser is using a piece of foam that is blocking up to %50, %50! of the vents causing a restriction in the frequencies responses with the HD 555's.
AOTS coverage: http://www.g4tv.com/videos/51377/The...derwood-21811/
How to & source: http://mikebeauchamp.com/misc/sennhe...to-hd-595-mod/
Congrats you just saved $150, if you bought the 595. . . .
Sennheiser:
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