Hello,
I don't have access to such a Siemens Acuson X300 hardware, but just a rough guess.
If your X300 only lets you set the network address and the AET of a remote modality, maybe you could try to make Orthanc listening to the DICOM default (port 104).
This can be done by modifying the "DicomPort" option of the Orthanc configuration file:
Anyway, this is typically the kind of information the paying support of Siemens will gracefully provide you. Do not hesitate to get in touch with them.
HTH,
Sébastien-