could a different power cord cure sibilance?

I think a much simpler explanation is that the capacitive and/or resistive loading of the ARC needs to be adjusted for your cart. I suggest posting on the analog board for advice, giving your cart model and the current settings on the ARC.

Thanks, I will try all of these suggestions. I use a Mint fof alignment, but loading and VTA are still mysteries to me.

Lloyde if it gives you any comfort you are not alone. I switched from the phono stage built into an ARC SP 6B to a supposed giant killing Graham Slee Amp 2 SE.I too have sibilance whereas I didn't before.
I can't fault the power cord because the Graham uses a wallwart step down transformer. (That sounds like something very fancy -hi end and seriously important - for a wallwart!) I'll bet that switching your cords around won't make it go away.I'm guessing that you might find a minor ameleoration but not an eraser.
I was also told to change the loading . I don't know exactly what that means or has to do with sibilance.The little phono box might have some dip switches but certainly no external control over the load. I was told that the "load" was the resistance in Ohms that the cartridge outputs to. As few as 47 for a real low MC and 47,000 for any MM.I am clueless about the other parameters.
I bought the amp 2 SE because it was suggested by the manufacturer for HO MC carts. I have a Dynevector 20X HO it's an MC. I suppose that it is meeting an MM load of 47K. This is clearly the root of all evil that I hear from my analog rig.
Good luck on your path ending in a Manley Steelhead with it's myriad of adjustable parameters.

New phono pre? Check electrical cart loading. The VTA is a mechanical adjustment.