Any particular reason that you don't use the constants 

System.Security.Cryptography.Xml.SignedXml.XmlDsigEnvelopedSignatureTransformUrl
System.Security.Cryptography.Xml.SignedXml.XmlDsigExcC14NTransformUrl
System.Security.Cryptography.Xml.SignedXml.XmlDsigExcC14NWithCommentsTransformUrl

instead?