Zanden DAC noise problem

I am looking for advise on how to solve the noise I get at high sound level from the Signature Dac which is connected to the Zanden transport.The dac is feeding an Einstein preamp driving Atma-Sphere M-60 which is powering a pair of ClassicLoudspeakers T3.3 .The sound is wonderful however when I increase the volume beyond my normal listening level there is a hum .All the other sources,Accuphase800/801 and Meridian861V6, into the Einstein are perfectly quiet.I am not in the mood to send the unit back to Japan,done that in 2008,they did a wonderful job but expensive and takes forever.
I have replaced all the tubes both in the power supply and the main unit,NO ,difference.I have also tried different output on the DAC,aes,and the cat cable.The cat5 works best( Have just replaced with Cat 6)I thought mistakenly that the the original Zanden CAT cable was defective,I have sent it for repair to US vendor unfortunately he will not return the cable nor respond to my requests.However good the Zanden equipment is,the technical support is not commensurate with product quality and price.All help will be greatly appreciated.