Where to use aftermarket power cords?

What is the most optimal use for aftermarket power cords? Preamp, amp, or digital equipment?

I think a reasoned approach would be to purchase a cord of your choice and try it in your system in each of the components you mention. The impressions you get will be based on your experience with your system....not anyone elses.
In my opinion, they're equally important. It really depends on how the power supply design in each unit.
The previous answer is correct as far as it goes, but I don't think that's what you're looking for right now. When I replaced my PCs I started at the source. I enjoyed the improvement so much I continued all the way back to the amp.

I tend to think the maximum improvement will be recognized by starting at the source.
I'm guessing that you have the budget for a single good cord and want to put it where it will do the most good. You can certainly have fun swapping around, and draw your own conclusions, but IMHO Nrchy is on the money. With this particular upgrade, the most improvement in my system came when I put a high-end power cord on the digital source. (With transport and converter separates, the DAC is usually the place to put the cord, but in one of my setups the opposite was true.) The preamp comes next in priority after the source. Changing the cord on other components may have a less noticeable effect.
I found that I got the LEAST impovement using it with my digital front end.

I have had two different digital front ends; a Krell/EAD combo and now a Marantz. The result was the same with both.

The rest of my Krell gear; preamp, x-over, and amps, were all greatly improved.

I noticed that my responses to Audiogon threads have been much more insightful since I started using an aftermarket power cord on my PC. My spelling has improved also. I don't know of there is a similar effect on Macs.
Well, I am a Mac user and after reading Thsalmon's post I thought that i wuld try a oopgrdad power krd on mi komputr soi treyed it and luk whut 343 2#$%# 5$b ???*>%F>!!!!

So I put it back on my CD transport and I'm much happier. By the way I've tried upgraded cords on all my gear and the best improvement by far was after I put them on the digital front end, especially the DAC.

Go figure...
When I started making my own PCs I had my old 'store-bought' PCs laying around. Just for grins I put one on my monitor. It did look better, the picture was clearer. It was not important enough to me to keep the cord there so I sold it, but they do improve monitor performance too.

To clarify my previous post; the better PCs improved every piece on which I used them. The improvement was biggest on the front-end though.
Certainly has made a difference for me on all equipment.
You don't realize that there can be blacker backgrounds until you get it from an upgraded PC.
Have one on my integrated amp (MF A300); and on my sources (Pioneer Elite DVD/CD, Cary 308).
I think it may depend on the actual construction of the power supplies and power delivery of each component as far as the amount of improvement, but I would recommend using one on all of your equipment except a subwoofer which probably does not need it.

No need to spend huge dollars as the upgrade from stock 18ga cords is significant usually with any aftermarket cord.

I started out using DIY 14ga Volex PC's that cost me about 10 bucks to make. Nice improvement all around.

I then splurged and switched out to Ven Haus power cords for everything which are less than 100$ a cord pre made (or you can do DIY with his materials) and they really allow the equipment to sound their best IMHO.

I am sure there may be are better cords than what I have, but for the money the VH cords are a great value.
Is there a definitive article from an unbaised source detailing the benefits of aftermarket power cords on an audio system, or is the improvement in system performance really just one's subjective perception? I'll have to admit I'm dubious, but I'd welcome a link to any articles on the subject that aren't sponsored by a cable manufacturer.
Lots of magazines have reviewed aftermarket power cords. Check the archives at http://www.stereophile.com. What difference does it make what someone else thinks though? You can get a PC for under $100, listen for yourself.

I quit subscribing to stereophile about eight years ago when all the talk about PCs started. That was just too much for me. A couple of years ago they sent me a subscription offer that was real cheap... so I thought what the heck.

They were still talking about PCs so I thought I'd find out what all the noise was about. I bought a cheap PC and hooked it up. The difference was immediate. I moved on to better cords before going to DIY cords from Chris VenHaus. I have them everywhere I can put one now and they are well worth the cost. That's the expereince of a former skeptic.
Nrchy, thanks, I'll check it out.
Instead of relying on the opinions of others, why not try some cables on both your source and amp from the Cable Company. They have a cable library that lets you hear the results in your system, with your music, your room acoustics and most importantly, your ears. Their website is www.fatwyre.com - check it out!