Hi hork,
Both the 34401A and the TDS 754C are very common instruments, and are widely used with our GPIB products. I am glad that the DMM works with the GPIB-USB-B, because that indicates that the GPIB-USB-B is working properly. Have you checked all the settings on the 754C to ensure they are set properly? Also, do you have another GPIB interface that you could use to try to detect the 754C? It could be a simple issue of a malfunctioning instruments.
john
Applications Engineer