could a different power cord cure sibilance?

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.

What Daverz said X3! Even more so if you are using a moving coil cartridge.

It appears that your EAR had an input impedance for moving coil cartridges of 515 ohms. See the last paragraph under the sub-heading "The Inside" in this review: http://www.audiophilia.com/hardware/Analogue/reviews/ear834p.htm

Your ARC phono stage has a default input impedance of 47K (47,000 ohms), assuming it wasn't changed during the modification process. http://www.audioresearch.com/ph3.html. It includes provisions to change that value, I believe by inserting resistors of the desired value internally.

Moving coil cartridges (assuming that is what you are using) usually require a low-value resistive load (tens or hundreds of ohms) to dampen what would otherwise be a high frequency resonant peak. That peak is often at inaudible ultrasonic frequencies, but associated with it would be a frequency response rise in the upper treble, that would, among other things, over-emphasize sibilants.

This paper is somewhat technical, but is worth skimming through: http://www.hagtech.com/loading.html

So the bottom line, assuming you are using a moving coil cartridge, is to change the resistive loading to around 515 ohms or to some other value within the cartridge maker's recommended range.

If it is a moving magnet cartridge, then load capacitance becomes important, as explained in the Hagtech reference, which would also have effects on the upper treble, although to a lesser degree under typical circumstances.

Regards,
-- Al

1. Maybe the new component has subtly changed the tonal balance ?

Sometimes a leaner bass may also manifest itself as a more forward overall sound, bringing into sharper focus other tonal characteristics.

Experiment with a little larger Toe-out on the speakers
This will also create a wider sound stage.

Also try moving the speakers back a little towards the wall / corner.

Careful speaker positioning can do wonders, and IMHO is one of the most neglected solutions...