Double check your configuration file includes the correct information for the Web service.
Try changing the URL behavior to dynamic as well.
Also, as John stated, I'm assuming you're adding the service to the class library because you intend to use it from the library, as opposed to other areas of the Web application.
"the credentials don't seem to work, and the call fails"...can you give a small stack trace of the error?
Just to clarify, in my current project, we use WCF endpoints within a class library with bindings and credentials. The same can be done for a SOAP ASMX Web reference as you're attempting.
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