I did NW to SIN about a year ago, and it was fine. The plane for the NRT-SIN leg was a new A330; options for getting to NRT include some A330s from the west coast, or a 744 from MSP or DTW.
The connection in NRT is quite easy. Don't know how it compares to the KE connection in Seoul, but it was one of the more painless connections I've had in quite a while.