Can upgraded jumpers improve sound?

I fourth jumpers. Clarity goes up a notch and the highs are much better. We have three NAD C320BEEs with various jumpers replacing the stock things. Joe Mazzaglia of Auricle Audio Design was kind enough to make me up a 6-inch pair when I asked, and these are the very best I have. The others use Kimber wire and they are also very good ; between silver and copper, the silver seem to suit that amp best.

I have a NAD C372 that I first replaced the metal jumpers with the standard Audioquest jumpers. The short black one for about 25 bucks. Vast improvement. I now use a .5m Audioqest Diamondback Cable for this. The diamondbacks were not much improvement over the standard Audioquest Jumpers but an improvement. I also have a NAD M3 using a .5m Audioquest King Cobra in place of the metal jumpers. So my answer to you is that you should definitely replace the metal jumpers.

Yes. You can use jumpers or standard cable, I use a standard length cable on my wife's NAD 3240. After all you are connecting an amp and preamp. Shorter is better but if you find a standard cable you like you can use that. You will hear a significant difference.

I used to use my NAD integrated amp with upgraded "preamp jumpers" that were custom made by Signal Cable. A significant improvement in sound which is noticeable. I could hear more details and the sound stage was much broader. Silver jumpers are better with NAD. I highly recommend them.
Currently my NAD is getting used as a power amp with a Promitheus TVC.