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For some reason they remind me of the RE-400's , it might be the packaging?
as always, excellent review. would you consider them an upgrade to KZ-HDS1?
Yes, I'd consider the M4 as an upgrade from the HDS1, however a rather small one. That both have quite different tonalities should be taken into consideration as well, as the M4 is quite a lot brighter than the HDS1.
another quick question, are they sidegrade or upgrade from from re400?
No idea, haven't heard the RE-400 myself yet.