The B&W 805N were very nice speakers, but they could be on the bright side - which may explain your loss of some "openness" in the vocals with the Kudus speakers. Do you still have the B&W or Yamaha? You could try swapping either/both of these items into your current setup to see. The older Yamaha might be built better than the very small Nuprime amp. Power supply is somewhat limited in the NuPrime. This generally affects how much current is available to drive the speakers. In your case, you indicated the Kudus are relatively easy to drive, so it is probably not a concern.
Based on your comments, you’re getting good volume, but not getting enough resolution and soundstage depth. I do think power cord and fuses can help tremendously here. In your situation, I would get some Furutech FI-15 Plus (R) Rhodium connectors for both IEC and Male ends and attach them to your stock power cord. These Furutech connectors are generally $56 each. The Rhodium plated copper elements will really improve resolution as well as bass punch. I have used these particular connectors in several situations and they are excellent for the money (same rhodium/copper elements as there more expensive connectors, so it makes them an excellent value). If you wanted, you could spend $20-40 on some DH Labs power cable wire if you wanted better than the stock cord.
Combining this with a rhodium Furutech fuse will really help smooth out the sound if the stock fuse on the Nuprime amp sounds too bright. This can happen sometimes with Rhodium power cord connectors as they are very fast and transparent.
Keep in mind that the Rhodium plated stuff will require 150-200+ hours break in before sounding best.