I seem to have better luck than most at getting the almost all of the "vestigial" hum out of my MC preamp -- nevertheless, while looking at some commercial dif amp data sheets I came across this circuit on TI's INA105 website -- the cable shield is fed to the inverting input of the diffamp. The non inverting inputs are fed from the output of the inamplifi's first stage. Should be good for low noise measurement.
