Quoting from myself in this current thread:
Regards,
-- Al
I have an older pair of Stax Lambda Pro's (non-Signature) with an SRM1-MkII solid state amplifier. I've found that ever since they were new, 20+ years ago, they have needed to periodically (perhaps once a month) have a severe "workout," playing at least a full album at volume levels that are higher than I would want to have reproduced next to my ears. Otherwise they sound very thin. I presume that the diaphragm needs that periodic workout to maintain the flexibility it is supposed to have.This effect is NOT subtle. I have no feel for the extent to which it may apply to other headphones. Note that the STAX's are electrostatic, while your Sennheiser's are more conventional dynamic headphones.
Regards,
-- Al