[G Georgopoulos]

I was informed its digital equipment that most benefits from shielding.  Be that as it may,  NuForce always provided a very nice cord with a choke.  I believe NuPrime may have continued the same practice.   Digital needs a bit extra help in that area. So far, I prefer shielding over a choke.  But,  I may learn otherwise over time. 

suppose the power of an amp is 40W
the current through the power cable is 40w/240v =166mA
most power cables are 10A
.166/10 x 100% = 1.6% is the amp current out of 10A cable
too much cable capacity is been wasted
now, a better cable will make much of a difference?

[Speedskater]

It's more likely the digital or SMPS units are radiating the interference.
* * * * * * * * * * * * * * * *
Jim Brown writes on SMPS interference:

Switch Mode Power Supplies:
  If you must use an SMPS, plan on adding suppression to it.  Start with common mode chokes on both the 120VAC and low voltage DC lines for the frequency range(s) where you hear noise. Capacitors across the DC output terminals and the AC input terminals can also help.  Select capacitors for low ESR (equivalent series resistance) at the frequency of interest and on the AC line side, use only type X1, X2, Y1 and Y2 capacitors, which are specifically rated for AC line use and designed to withstand the 3-6 kV spikes that can occur on power wiring. Choose this capacitor carefully – if it fails, it could catch on fire!

SMPS units tend to be noisiest at lower frequencies,

[Genez]

suppose the power of an amp is 40W
the current through the power cable is 40w/240v =166mA
most power cables are 10A
.166/10 x 100% = 1.6% is the amp current out of 10A cable
too much cable capacity is been wasted
now, a better cable will make much of a difference?

You're looking at the wrong things possibly.   Many will tell you how much better it sounds with an audiophile cable that was made well.   I do not believe in spending hundreds of dollars.  If it were not for Pangea power cords ( http://www.audioadvisor.com/products.asp?dept=25#filter:brand:Pangea$2520Audio ) I would scoff at what some pay hundreds for.  I would never bother to find out.  And,I would have lived in an antagonistic attitude towards them, because they would be seen as toys for rich people.   

But,  I will tell you this.  After seeing what a good difference they can make?   It opened my mind to venture into things like simple fuses making an improvement.  And, they can. Get the right combination, and they can transform what you hear in a surprising way.  And, the most expensive do not have to sound better for your equipment.

[Genez]

It's more likely the digital or SMPS units are radiating the interference.
* * * * * * * * * * * * * * * *

That is one valid reason why shielded cable can improve what you hear.   I use a shielded cable on my audiophile computer.  And, having one on a DAC can prove helpful. I just found that the most reasonably priced power cords are good all around,  even for analog components.  It just works.  I am not concerned about scientific data when my ears tell me its OK.  Now,  those so inclined who are curious as to why it sounds better?   I wish them luck. Even some folks running high end audio companies do not understand why they sound better.   They just know they do, and can only offer theories as to why.  The fact remains.  They do sound better in many cases. And, they do not have to be ultra expensive. 

[Syrah]

Chris Venhaus' rule of thumb is - use shielded power cords on digital equipment; use unshielded power cords on analog equipment.  I think he bases this on extensive listening tests with his many designs.  That seems consistent with your the above theories, though I suspect it's a YMMV situation depending upon system setup and how bad/good the power designs.

I swore by shielded ICs, but I've been slowly replacing them with unshielded ICs, just based on doing A/B listening tests, sometimes blind by asking a friend (or wife in a particularly patient mood) to swap them for me and not tell me which is which.

Side note on grammar - I'm somewhat over-educated, but I often do need to think twice about "effect" vs. "affect".  Thanks jeffreybehr, that's the simplest most elegant explanation of the difference I've ever heard, at least for when used as a verb.  I'd post that on my office cork board, but for the embarrassment, perhaps just a note in my wallet.  I appreciate being corrected.  My PPs are the constant misuse of "literally" and people using "may" when they mean "might" because "may" sounds fancier even though it's wrong,  Just the other day I heard on the news about how so and so "may" die.  Yeah, well we all "may" die.  News flash, someone is not immortal.