exchanging hard-wired power cords?

I'm considering upgrading the power cord on my NAD 370c integrated amp. Should I leave this to a professional or
can I do it myself? I've done minor soldering repairs for
many years on instrument gear, but not HiFi. Thanks.
You should be able to do this yourself. Remember to use the best eutectic solder you can find. Cardas eutectic solder works great.
In my experience, replacing a hard-wired power cord isn't difficult as much as it is messy. If the replacement cord is thicker than the stock cord, you might have to drill a bigger hole into the chassis and then get a larger grommet. Another option is to remove the old grommet and not replace it, thus allowing sufficient clearance for the new cord without drilling a bigger hole. Of course, using the latter approach, the power cord won't be well secured so you'll probably want to do something to insure that the cord stays in place and is protected from the metal edge of the chassis. A simpler possible option is to enlarge the inside of the grommet by filing down some of the interior of it, and then using it with the larger cord.
Have you looked inside? Often the power cord has a smaller plug than the normal one on the outside. You would then just need to find an adaptor. I have a real nice CD player new cord but the plug won't fit into the one inside. The one inside is smaller. If this is the case, and you are able to fing an adaptor, I would love to hear about it.
When replacing a cord, it is important that you maintain both proper polarity and safety. While altering the polarity of the power cord could incurr a safety hazard in itself, i'm talking about adding some type of "strain relief" to secured the cord within the device. Otherwise, it would be possible for one of the wires to break free from the circuit board due to rough handling, snagging the cord while moving something else, etc... If such were the case, the "hot" wire could come free, short out the unit, start a fire, shock / electrocute you when attempting to handle the chassis, etc...

Since most replacement cords are larger than factory cords, you may end up removing / negating the factory strain relief / cable clamp. As mentioned above, you may even have to drill out the hole a bit. The main thing here is to make sure that the metal chassis is free of burrs / sharp edges. Once you've done that, soldered / attached the cord in place to the circuitry, you need to secure the cord in place. The cheapest / easiest way to do this is to use a wire tie on the inside and outside of the chassis. This keeps the power cord from being shoved into the box and at the same time, minimizes the chances of the cord being yanked right out of the box. If you want further protection, applying a layer of "hot melt" or "hot glue" around the entire power cord where it meets the chassis inside the box will both reinforce the holding power of the wire tie and actually anchor the cord to the chassis. This is a "generic" method that works well and is completely reversible in my experience. In most cases, the "hot melt" can be peeled away from the chassis and power cord with no visible signs of damage to either.

As a side note, BE VERY CAREFUL when drilling into a chassis. Besides the potential for drilling into circuitry / wiring, you will also produce loose metal shavings. Not only do these shavings have the potential to short things out, they can find their way into moving mechanisms and do permanent damage. ALWAYS clean a unit out very thoroughly after opening it and doing ANY work on it. It is a simple precaution that just might save you a LOT of money and grief. Don't take for granted that you did everything "right", cuz all it takes is ONE mistake to negate the benefits of a new cord and give you a major headache / heartache. Sean