When I owned the X350 the best cord I had on it was the Elrod Sig 3 and in a fairly close second was the Kimber PK-10 Palladian great cord for the price all though you really don't see them up for sale.. I tried the Virtual Dynamics nite, really didn't care for this match up?? maybe it was just my system? Purist Audio Dominus, another great power cord. Cardas Golden Reference, BMI, Silent Source SIG, AUDIENCE, JPS Labs ,Harmonic Tech AC 10 and so on .
I have to second bigkidz the Elrod Sig 3 is just a great match up with the X350 and X250 for that matter I had both.
If you can find a used kimber Pk10 it would be a little less$$ and also well worth it. Good luck.