Can upgraded jumpers improve sound?

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.

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 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.