could a different power cord cure sibilance?

recently upgraded my phono pre to an AR PH3SE modded by Great Northern. In a different league from its predecessor, a stock EAR 834P, but now there is too much sibilance on some vocals (e.g., on Thriller or MFSL Amercian Beauty). Definitely caused by the phono pre. I understand that the tubes used could contribute to excess sibilance, have tried various types, and will try others. Could the power cord affect this too? It's about the only other variable. Currently using Signal Cable Sivler Resolution pc. Recommendations welcome.