Darkkeys,
NAD told you what their lawyers told them to tell you, whether it is the truth or not.
As for cords, if you believe there is any truth to the fact that a cord can make a difference (I do), here is what I suggest before you invest $500+++ on a new power cord for your already wonderful $800+ integrated amp and your modest digital sources.
A very smart man who also happens to sell hi fi gear told me about these tweaks, and after trying them for myself, I didn't buy any of his gear but now really like him :)
1. Make sure your speakers are set up properly - they should be at least several feet from the walls beside and behind them. They should be resting on the floor on a very sold surface like a heavy board, piece of marble, concrete, etc. and isolated from the support by blutack or spikes (this is one of the most effective tweaks I have tried yet). If they are monitors, they should be on solid stands and the stands are otherwise isolated and set up as described. If the tweeters are not pointed at your ears in your listening position, tilt the speakers so that they are. Make fine adjustments on distance from walls, toe in and lateral separation to taste.
2. Make sure your electronics equipment is protected from outside vibration, and able to dissipate internal vibration effectively away from itself - especially your digital and analog sources. Place them in a good equipment rack AND each unit on large pieces of maple or other hardwood (this is also one of the most effective tweaks I have tried yet). Try brass or other spikes between the equipment and the wood - avoid sorbothane or other pliant material between the source and the wood block, but are OK underneath the block to reduce external vibration.
3. Are you running at least decent interconnects and speaker cables?
4. I would look at spending up to $500 on a used CD player like a Music Hall CD-25 ($300 on Agon now), Jolida JD-100 ($600 on Agon now), Cambridge Audio Azur 640C Version 2 (just sold for $325 on Agon), etc., before spending it on new power cord. You can't add something back in the audio chain that doesn't get off the disk, out of the DAC on the way to your amp and speakers in the first place. The Marantz will play a lot of disks, and the Oppo will play a lot of kinds of disks, but neither one is going to drive the NAD to its full sonic capacity (although I would suggest that the light and somewhat thin case on the Oppo might benefit greatly from a large block of wood beneath it - and a big chopping block is a lot cheaper than $500!)
5. Now that the rest of the audio chain has been physically optimized, you have a better chance to see what improving the power situation can really do for you. There are some synergies here that makes me not exactly sure what is going on.
First, replace the cords on your computer or other electronic devices, that you might leave on and know draw a lot of current, with your audiophile cords to speed break in. Next (>150hrs of burn in) try to unplug the power conditioner and drive your amplifier and sources directly with different combinations of the audiophile and stock power cords you have on hand DIRECTLY FROM THE WALL SOCKET. If your home power is not just crap, you may start to hear big differences in the sound the now broken in cords provide in your system. Typically, hefty gauge wires (smaller number, bigger diameter) work better on a high current amp like the C372, while lighter gauge (larger number, smaller diameter) and well shielded wires work better on digital sources. Experiment directly from the wall socket with the broken in cables and see what you like best. If everything in your system is set up properly and your power supply from the wall is half decent, you should be able to discern differences between even modestly priced but well designed power cords and the stock units that came with your electronics.
If you are experiencing hiss or other background noise problems, you may want to reconnect your power conditioner and experiment further. Power conditioners are tricky and most less expensive ones tend to deaden and smooth out the sound, clipping peaks and transients off the signal (BAD!). For this reason I would only use them if I was suffering problems with dirty power resulting in a noticeably high noise floor. If you identify that you have this problem, then investing in a very good power conditioner that has the least sound signature possible may be necessary and more important than an expensive power cord.
For what it is worth, I am lucky enough to have really clean power in my house because I am very close to the electrical pole, and I replaced all the wiring in my house 5 years ago (not for audio reasons, but it is a significant side benefit). Replacing my budget power conditioner that I thought I needed with a $50 Wiremold super minimalist powerstrip made a huge difference in my system performance, especially on transients. Replacing the stock 18 gauge cord on my Cambridge CDP with a simple 16 gauge shielded cable from an old laser printer made another big difference(!) I listen frequently through headphones and the system is dead quite, even with no signal and the volume turned way up.
Good power cords work by providing adequate current flow and at the same time shielding interference induced by relatively high current and digital noise, via clever geometry or exotic materials, from influencing your other electronic circuits and the low level signals going to the amp via your interconnects and the speakers via their cables. Power cords and conditioners should be noticed by what they don't add to or subtract from the musical signal running through your system. The better they do this job, and the better your system is optimized to reveal these improvements, the more you will notice a difference. If you still don't notice any positive benefit, then you saved yourself some money.
PS - The Cable Company http://www.thecableco.com/ offers demos on cables that you can try in your home with your system. Good luck.
