From what I have come across, Napster music that is downloaded as part of its subscription or purchased is encoded as 128kbps WMA version 9. Napsters Janus subscription which allows music to be transferred to a Janus compatible player uses 128kbps WMA version 10.
The only thing I am not sure sure is if they use single pass or dual-pass encoding. Assuming they use single pass encoding, the best way to see what its quality is like is to encode music (from an original CD) into 128kbps WMA using Windows Media Player 9 or 10. From what I recall, Windows Media Player 10 encodes music in the version 9 format. Only Windows Media Encoder (or other WMA encoding software) gives the option of dual-pass encoding.