Not familiar with Dynaudio warranty policy specifically, but you generally need a receipt, registration or some dated proof of purchase to obtain service under warranty from any vendor.
If not under warranty, or dated proof of purchase is not available, then I'm sure you can still receive service, which in this case probably means a replacement driver or shipping of speakers to a service center, but you would have to pay.
Or, if you think the speakers had the issue when you acquired them used, you might take it up with the seller to determine if warranty can be established or some form of compensation if the condition was not as advertised.
If not under warranty, or dated proof of purchase is not available, then I'm sure you can still receive service, which in this case probably means a replacement driver or shipping of speakers to a service center, but you would have to pay.
Or, if you think the speakers had the issue when you acquired them used, you might take it up with the seller to determine if warranty can be established or some form of compensation if the condition was not as advertised.