could a different power cord cure sibilance?

No, IMHO. Why not hook up the stock cord or swap between another component to see if there's any difference?

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.

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

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.

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...

As I recall, the Dynavector 20xh is designed to work into a 47k load. Mine does.

You might want to let the new phono pre breakin for about 300+ hours before you change anything. Every time you try different tubes you have to give them time to breakin. Even if you put the old ones back in after a few days again they still have to breakin. Location to other equipment can also cause issues you discribe, because it has a high gain stage. Are the wires routed the same way, the ones coming from the arm to the phono pre could be picking up external noise.

Thanks for all the advice. I think Tvad is correct, the new pre revealed pre-existing flaws, particularly in certain recordings; no doubt Mechans is correct that in the end, a Steelhead or something equally adjustable will be required. I am using a low output Grado Reference into a 47K load. The Grado is supposedly insensitive to loading, which is why I didn't initially consider that a likely culprit. Nevertheless, as suggested above, I am trying different loads, starting with 800k. A pain, as resistors have to be soldered in (short term, a little less trouble than making some plugs) and, I assume, burned in for a while. 800k hasn't really solved the problem. I also realigned everything (with a Mint), messed with VTA, and switched power cords; all of that may have helped, a little, but not enough. Scouting various forums, some say the Grados are known for sibilance, but it wasn't clear if that is true of the higher-end ones (although, I was surprised when a distributor referred to the $1200 Grado as a "budget" cartridge. Try explaining that to non-audiophiles!) I will be asking for cartridge recommendations when finances permit.