could a different power cord cure sibilance?

Better power delivery usually reduces hash, but this is what the GNSC mod is for. Sometimes a different power cord can also reduce hash, but not all power cords work the same way in all systems, so Cruz123's suggestion is a good one.

I generally consider sibilance different than hash. Sibilance comes and goes depending on the recording, whereas hash is always present regardless of the recording (although varying dynamics will reveal or hide hash).

IMO, the GNSC mod is exposing a pre-existing front end flaw, and/or it's exposing flaws in the recordings which have sibilance issues.

I'd consider looking at adjusting cartridge loading, or set-up, or the stylus profile. I noticed a reduction in sibilance after I set up my cartridge with a MintLP Best Tractor, and when I changed from a Benz Micro Ace to a Dynavector XX MK II cartridge.

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.

What Daverz said X 2.

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.